It's the craziest question, but I like to do things properly...

What's protocol for overhauling an entire costume? Like, I'm re-making it from the top down, so I need a new shopping list, new to-do list, this, that, and the other. Is it 'the done thing' to add a completely new 'in progress' costume to my account, or should I just modify the old one?

Told you: Crazy question, and don't bite the newbie, but it's really stressing me out xD

I'd say it's totally up to you

Some people edit and some start fresh. It depends whether you want to keep a record of the first attempt, I guess.

Like puzzledpenguin said, depends if you want to start a fresh or keep a record of what you've done so far.

I'd say depending on how you feel about it before remaking, will affect what you do. If you dislike how it turned out, just using the exiting stuff but deleting photos and just adding from there with journals and lists would work. But if you like it, just want to imrpove upon it or use better materials or methods, then make a new thing for it and go from there.
I personally would make a second one, but call it MK2, specially if it was a bigger cosplay. Smaller ones or ones I wasn't as proud off to start, I'd likely just update the original files.

Yep. Personally I'd do it as a new costume too. It's not like you're modifying the existing one. Starting afresh is a new costume.

And later you can compare the trwo and be impressed at how your skills have improved

There are plenty of people here with repeated costumes. If you're totally revamping it, then add a new one. If you're just making some little improvements here and there, attaching that bit of lace you didn't have time to do for example, just edit it :3

Usually people start it as a new costume and refer to it as 'version 2' or 'attempt 2' or whatever. Its no big deal, do it how you want. The way you present your cosplays is your own prerogative. There's no "right" way to go about it.

Thanks for the reassurance, guys! I'm just forever paranoid about doing something wrong xD