My normal group's DM is no longer Dming for us because reasons. I've taken the role for dungeon master since i have experience Dming before. I wanna change a few of the homebrew rules. But i've never seen or heard of Dms swapping mid campaign and I need a little help with how to swap them. Should i hold a session where we go over new rules and rules that we want to change or drop (like use coin weight). Or should i just roll with the Rules as they done laid out.

Rules i want to change

Use coin weight to ignore coin weight. Being able to have 4 sword (2 on hips and 2 on the back) to the normal 2 swords or 1 sword and shield etc. Poison lingerer for a 1d4 (plus a 1d4 depending on strength of poison) days to they disappear in an hour or 2.

If one DM has departed and another player is stepping into the DM role to continue a campaign, a pause session is in order.  Life happens to people so your situation isn't without precedent.  In your session basically review how the game's been run and how you intend to run it.. Since this is an established campaign I'd suggest running your desired changes by the table and make sure you have a consensus before just imposing a bunch of house rules and flipping a bunch of existing house rules.  Yes, you're the DM, but you want to have a good dynamic with your table and not just show up and poof some mechanics have suddenly change.  This non session should also review the themes and content your adhering to, and lastly you should ask the players if there's anything they all think might be interesting to try out for bettering the existing game.  

Lastly, when you do DM, make sure at least for the initial sessions, you build some time in to end game play proper and ask them how you're doing.f

As for your specific changes, I'm a little lost as to how a game that used coin weight allowed characters to carry up to four swords.  If the players like encumbrance stick with it if these odd carries work.  

Thx for the info and is was mostly everyone threw the gold into a bag of holding so only 1 person held the gold aka the Wizard who never needed to use melee as much as a rouge and etc.

I’d agree with plat about a pause session. Basically have a session 0 like you would at the start of the campaign. Talk about any house rule changes is reasonable. For things like an encumbrance change, you should probably give a grace period, if they’re in the middle of a dungeon and suddenly there’s all this extra weight, you can really screw them. Wait until they’re in a town where they can find a way to unload extra gear or otherwise get themselves together. This is also the time to evaluate any homebrew classes or races, and make sure no one has any items that make them OP. Just because the past DM was ok with them doesn’t mean you have to be  

You also could use it as a chance to check in with the players about how things are going. Make sure they are still happy with the direction and style of the campaign. Presumably everyone was on board with it when you started, but they may have changed their mind and this is a good chance to make sure. And now that you’re running the game, you may want to change that to some degree, so it would be good to work through that as well. 

Yep - I'd also suggest a session 0. Discuss the rules you'd like to change up with an open mind with the players and be aware of anyone whose character appears to rely on the rules you want to change. (Given the ones you listed that isn't likely but you never know - it sounds like having 4 swords is something of an anime type character image and so more for appearances than for mechanics but it is hard to tell).

Also, if you plan to change up encumbrance, then just make it clear what the changes are and whether it will negatively affect anyone in the party.

As for poisons, I'd just change it however you want though if the characters already have samples of the older poisons then let them use them up and just say that those poisons used rare herbs that no longer appear to be available. There are lots of ways to deal with poisons.

However, I would make a list of ALL the rules you want to change and make sure that it isn't too long and that the players don't mind the changes.

100% backing up people saying finish this particular part of the campaign and then discuss with the players what they want to include. Carrying 4 swords at once in ready positions is something of an odd desire - how does this work with getting through doors? Having 2 swords crossed on your back and another 2 swords on your hips is going to make it quite awkward.

I also don't see the synergy of these requests - making gold have a weight is a push towards realistic, having poison linger for a while is a push towards realistic, and then there's a guy swinging 2 swords around with 2 more swords on his back which are probably getting in the way of his swings. Are you putting anything in place for the people who don't want multiple swords? what benefits do you see from having multiple swords - the only way I can see it happening is if the user has several magic weapons, otherwise they will probably just have longswords for the maximum damage from a one handed sword, and then there's no need for them to have 4 unless they are expecting to throw them or something! 

Speaking only for myself, I think I would only change rules if they have been a bugbear (see what I did there?) for the party and the previous DM was refusing to budge. Even then only if it was reasonable - you don't want your first act as DM to be rolling over to unreasonable requests that break your game. Really bad precedent to set for yourself.

Now you might want to change rules to make your job easier as DM. I'd only do this if running the other ruleset was really, really confusing and difficult. And I'd talk to the party first. "Listen guys, I know you're used to XYZ, but honestly that's awkward for me and I think it'll slow down the game. I'd like to run the rules as ABC to keep things flowing." Then I'd only change if they agreed.

Once you're through the campaign played as everyone agreed, a new campaign can be started with its own session 0, and you can lay out your house rules and everyone can decide if this is still the right table for them.

Yes redo Session 0.  I been in two types of DM switching. Shared World where the DMs divided up the world. Or each DM had their own rules and own campaign. Now you were unclear or I not use to usage of campaign. By campaign do you mean you picking up the book (Icewind Dale) and running from where the group last left off? Or something different?

it's a mixture of both luckily we are in a book (Tales from the Yawning Portal) but some of it is home brew open world.

the other thing I will add is have a private chat with each of your players about their character. There may well be character secrets or plans that where agreed with your old DM, or just to understand there characters as a DM. You have developed alongside them but that relationship in game is very different to the DM player relationship.For instance in my game every player has secrets that have not yet fully come out to the group. They all have backstories that I have fledged out on my own notes with the bits they don’t know (who betrayed one, another what there origin is (warforged), what the warlocks patron wants etc. 

Also has he given you any notes on order to help you tie up any loose ends he had left around or was the campaign left at a good end point. I would suggest house rules are probably less important then making sure your players don’t suddenly have a change of direction for their own characters that they are not doing spearing, or wanting. 

Quote from UmbreonNameMoon >>

it's a mixture of both luckily we are in a book (Tales from the Yawning Portal) but some of it is home brew open world.

 As long as you don't change any of the homebrew rules, you should be fine. I like running Portal but some my players were rushing through it, and I got blamed. But I had to deal with Adventure League drop ins.