https://wiki.garrysmod.com/page/Workshop_Addon_Updating

I think this would answer your question: gmpublish.exe update -addon "path/to/gma/addon.gma" -id "1337" -changes "Fixed something"

If people wanna see what's changed they could go over the changes notes thing in the addon and read whatever you put after -changes

That's not the thing. I could make an external changelog if I wanted. I meant that making models takes time and the best way for players to realize the addon's been updated currently is either to check its workshop page or see new models in-game.

You could make a group and make the events that the addon's updated? That'd be a awesome way.

That's not a bad idea actually. For now, I've got a progress board up on Trello:
https://trello.com/b/GHti1KFw

also you could keep up progress through updating google docs and putting new info in there