Sorry if this has already been asked. I've been trying for a while now to create some custom weapons that can transform into another weapon (dagger into longsword, handaxe into battleaxe, that sort of thing), but I can't figure out how to get the damage to change. Can anyone give me some help?

No offense, but why do you need it to?

Just put all the different weapons on your character sheets and use the mechanics to fuel the role play. Just put a hand axe and a battle axe on your character sheet and then say “my hand axe transforms into a battle axe”. 

You’re making it more complicated than you need to, in my opinion. 

And you will have to go with Cubester's suggestion for the time being, because weapons (and items in general) are currently lacking a bit in homebrew. Namely, items can't have options or add actions.

That doesn't help because now the player would have additional weight for weapons they don't actually have, and it would be more complicated to say "well I just subtract 12 off my weight because these items aren't actually in my inventory". I think it's a great idea to have all of the weapons in one, without adding to weight, etc. I just wish Beyond would add more expanded abilities.

Anyone who's a strength-based character is realistically never going to have to worry about the weight of their equipment anyway.

You can edit the weight of items in your inventory. That's what I do for my Pact of the Blade Warlock so I can have a bunch of weapon types available.

You could also add a Custom Action on the character sheet!

Yeah just edit the weapons to 0 weight is my suggestion as well.

Throw it all in a bag of holding & now it only weighs 5 lbs. Rename bag of holding to weapon of changing or something much more clever. 

Smart suggestion, would keep everything organised in the inventory.