(Skip to 'Now to the subject...' If you don't want to read my vent.

So it's probably been posted about a million times before but I need to vent.

So as the DM for a group of friends I have been having fun making props, making maps to use alongside LMoP,  that's great, its fun and I have fun running the game.

Sometimes.

I struggle to find time to be ready to DM for the group in a way that I feel comfortable, having side notes, mini scripts etc. (just to make my life easy). Now one player in my group was constantly saying that it can't be too difficult to DM, so we ran a one-shot. New Years Eve and he had prepared a Harry Potter one shot with 5e rules. Players lost interest, I played, didn't enjoy the experience.

Now following LMoP the players want to move on to Dragon Heist. I have the book, I have flicked through briefly and have sort of come to the conclusion that there is a lot leg work involved.

Now to the subject...

Just as we had finished setting up for the 4th session, one of my players announced that he and another were just finishing up his new character sheet. News to me. They both informed me that he would play the character for the session but he will be using the new one at the next.

Is that normal, do players usually just decide they are going to start a new character on the fly without even talking it through with the DM? The player who assisted is a bit of a meta gamer and thinks he is the be all and end all of D&D knowledge because he has watched nearly every episode of CR.

How would other DMs handle this? because it has caused me to have a mini meltdown.

simply say NO. at times you have to be "harsh" and consistent. before you know it they'll be making new characters every few sessions.
there are always players who have tons of character ideas. and they want ot make and play all of them, which is impossible.
later on, if the narrative allows, you can set it up so that the main group starts their own adventuring company. meaning their other characters could join and depending on how you setup the adventures/story elements they can occasionally change in/out of characters. if you want to go that far that is.

It is normal (in my experience at least) to brew and test build different character ideas, but definitely unusual to decide to play one without discussing with the DM first.

So now, I guess you need to decide when or how to incorporate the new character into the story, and discuss with the player what they want to do with the old character.

I would not allow the PC to dictate when they bring in a new character. Definitely allow for opportunities for PCs to swap but as the DM, it’s up to you. 

It is poor manners of a player to assume that they can choose to change something as dramatic as a character without the consent of, at the very least, the DM. 

I would tell the player that this new character can sit in the vault, then explain that their approach was inappropriate. I would then follow up with asking why they crafted a new character because, as has been pointed out, there may be more of a reason than wanting a shiny new toy. Depending on their answer you can follow up with a flat no, change some of the story up to match the RP needs that aren't being met, or discuss ways to adapt the current character to fit the mechanical play style they're looking for. 

In the end this type of action isn't indicative of someone who's respecting the table etiquette, and that type of behavior should be addressed as soon as possible. Worst case scenario this player, and potentially the accomplice, may leave. Your goal should be to hear them out and make any adjustments that you feel are appropriate, but they should be able to respect your decisions. If they're unable/unwilling, then they won't be a good fit for this current game/table. 

Regarding "the subject," I agree with DMThac0 above, and totally sympathize with your frustration. 

Regarding the venting you did, you're a little bit all over the place, but it sounds like you're feeling like your efforts are unappreciated. It is a lot of work to be the DM, and it could be a bummer if you're not getting the kind of "yes and" response you want from your players.

It also sounds like you're having trouble finding enough time to DM at your own preferred level, which is a separate problem, but one which I also totally get. 

It might be worth taking a session or two off and reshuffling your priorities, so you can figure out how to make the game work for you and everybody. A schedule change might help, for example. It also might help to read Dragon Heist more carefully, and see what kinds of decisions can be made to minimize the legwork.

Hearing from a player that they're making unilateral changes to the composition of the game is irksome, but I bet it wouldn't cause a meltdown if you had the other items worked out. Good luck!!

I have had players ask to change characters mid campaign but have never had one tell me they were going to make the change. I consider that rude and poor form from the player. I don’t mind sitting down and making alterations if a character in play isn’t quite what the player thought it would be or if it is a poor fit for the group/campaign. It wouldn’t hurt to have a chat with the player minus the help of your meta gamer. The problem could be an easy fix and misunderstanding about good game etiquette. But don’t hesitate to put your foot down. Meeting players wants and “needs” can put dm’s into the position of feeling like the proverbial door mat. Good luck.

I had a player talk to me about wanting to change her character because she was having a lot of trouble connecting to it. She'd been running that character for about 4 months, and had certainly given it a red hot go. So I agreed to let her, and we wrote out her old character and brought in her new one. Not a problem. I wouldn't want to make one of my players continue to play a PC that they weren't enjoying, but in your game, they definitely went about it the wrong way. It's a tough spot, because while you may normally say yes, the way they did it is almost certain to make you want to dig your heals in. I recommend against that. You need to have a good conversation with this player. First, ask why they want to change. It might be something you can resolve. They may not like how you handle their PC, or they may feel uninvolved in combat or RP sessions. They may feel like you aren't doing anything with their backstory. It may be fixable. If not, then let them move on. But discuss with them how they should have gone about it, then discuss how YOU are going to bring in their new character. They don't get to just wake up in the morning and change from a tiefling bard to a kenku sorcerer. For what it's worth, a nice clean way to do it is have the new PC being a hostage in some sort of situation and the old PC gets killed during the combat to rescue them. Although I have also had Neifion just tear a hole in the fabric of the realms and snatch someone through it. Shrug.

Unless the PC is somehow critical to the campaign then I'd say roll with it.  Who wants to play a game with a character you're not enjoying.  If you are anything like my D&D group we are all busy and don't get together as often as we'd like, so even one session can be a drag if you are wanting to try something new.  The reality is if the players aren' into what they are doing then they will quit the group and everyone loses.  So why make a mountain out of a molehill and lose any sleep over it.  I tend to make my adventures in a manner that is independent to the composition of the party.  As such, if everyone decides they want play a rogue, then we have 4 rogues in the party and they still have the same adventure in front of them.  The game continues just fine.

Having said that, it would have been more polite and respectful to the DM and the other players if there was some discussion first.  And if there are any other surprises they might want to share with the group.....

I normally offer new players or players that try the class the first time a "can I return this and change" policy.
I DM since 17 years and played a few times too as player. Sometimes your class or char concept sounded good first and after actually playing it or letting him walk with the group, you dont really feel comfortable playing that char anymore. At this times I let my player know they can always talk to me. 
There is a checkpoint in all my campaign where they can change their class/char and a point of no return. I let them normally know at the last checkpoint "this is the last opportunity to change class. You all sure after X Levels and X Sessions that this is the char you want to play till death take you apart?"

But never in mind, a player in my games ( I have like 5 Groups and a total of 28 players under my wings) would think to just come to me and say "lol xD next time I just change! Deal with it"

That player would lose his place in the group really fast (there are enough people in my area that want to join and it seems that only 3 other DMs are around here... and they are full like me)

Same with "I know it all, because I watch CR and read the book once". Those player I just prove otherwise (every DM has probably eaten the rule books by now because you need to look often into it) or just say "for simplicity, keeping the game experience fluid and logic I will just rule it over storytelling instead of using 2k mechanical rules that are just some random numbers". They can take it or leave the group. But this problem is not present (anymore) in my groups because I am more of a storytelling DM and everyone knows that errors can happen with rules, but in the end fun is more important

Further there should be a common respect between player and DM. The player should know from the start, how much work and effort a DM puts into a session. The DM should have an open ear for the needs of the players. Never should it happen that anyone thinks its a "us vs the dm" scenario. They should more think of you as their guidance in an adventure and not the enemy. If this is established I doubt that those problems you mention will even surface

Honestly since it's just running a module I say let it happen. Better than letting them kill their character off top force the change. If it was a homebrew campaign that would be a more difficult thing to deal with. Still doesn't excuse the player just announcing it without asking first. 

Also a side note I feel you maybe overthinking LMoP. It's got the major stuff already done for you. All that's needed is to ad lib some lines. 

And don't be too hard on the one player who tried DMing. They did a homebrew setting on their first try,  which is a terrible idea and should have been explained to them.

First of all, it sounds like you are verging on being over-prepared for your sessions. Props and maps and such are great, and they add another element to the game, but if between that and trying to make notes, NPC "mini scripts", etc., it seems like you're trying to make your life easier by over-complicating it, and driving yourself crazy. Not to mention, the players WILL invalidate your preparation by doing something off the wall, going somewhere you weren't prepared for, etc., it's bound to happen. Try to keep just the broad strokes of the story in mind, but let the players fill in the details, and take short notes on what they are doing. 

I also recently had a player who wanted to try his hand at DMing, so he took over for about a session and a half (we are also running LMoP, and he replaced the Wyvern Tor encounter with a small dungeon), and we worked it into the story. Don't expect a first timer to run a game like Mercer, but if your player is interested in learning the ropes, help him out, cuz it's fun to switch it up and be a player sometimes too!

Now, on to the point. To put it simply, NO, your players should not be telling you that they made a new character and plan on switching for the next session. This is definitely something that needs to be discussed with the DM first. Like some of the previous replies said, it may be that the character isn't working out like the player thought, they're having trouble with the mechanics, or some other legitimate reason for wanting to make a new character. This is fine, and of course we want the players to enjoy playing, but it needs to be worked out away from the table. Offer to help the player without the influence of your "CR guru", and figure out what he/she doesn't like about the current character, what they're hoping to get out of the new one, etc.. If you don't want to just tell the player no, tell them they need to come up with a good RP story that ties in the new character, and gives the old one a reason for leaving. Or, you could tell your player that they can use their new character if their current one dies, and watch with amusement at how they suddenly turn their character into a reckless maniac! Finally, you could offer to the entire table that after LMoP is done, anyone who wants to try a new character for the next adventure is welcome to do so, and the current party parts ways after the events in Phandalin. This allows the players who want to continue with their current characters, and the ones who maybe aren't satisfied with their character for whatever reason, the chance to change it up for a new adventure! 

It sounds to me like the experienced player thinks the party could use a PC that is another class and then talked this fellow into the idea of switching his character with advice on how he would enjoy playing this character class in this adventure rather than sticking with his original character.  Wild speculation on my part, but I think there is an idea between the two of them.

So I think just talking to the fellow and asking him why he wants to switch, as so many other folks said above, would give you the opportunity to address the situation.  "Address the situation" doesn't mean going one way or the other, just giving you, the DM, the flexibility to do what is right after talking to the player.  I would have this conversation one-on-one and not have the other fellow around.

Being DM is a lot of work and you deserve respect for your efforts.

How is Mr C-R at being DM?  Maybe let him run the next campaign and see how it feels.