Something like stats changes, gold, or items already in the game should be fine. Even mods that just change the graphics shouldn't affect your save file as long as nothing is moved to a new location.
A modded save file should really only break if you add new weapons, armor, enemies/enemy types, classes, or anything else not usually in the game. Likewise, it would also probably break if you start moving the placement of things.
At least, that's the tl;dr of how stuff like this normally works.
