I recall the one where Bowser was given Mario's moves and playbility, while the original Bowser was replaced with a giant fire-eyed Mario. 'Course, in NSMBW, the playable characters didn't really HAVE separate movesets...
We call those model imports. Pretty sure for that hack, Bowser was extracted from that and rigged to Mario's bones, then edited bones and animations for Bowser's proportions then reimported.
That's also just a model import. Unless they edit happen to edit moveset properties, all what you see in that Bowser hack is just a model and edited bones and animations.
I think Peachette is an interesting idea, and I find it hilarious that there are people talking about "lore implications" regarding "Peach just being a Toad with a powerup."
Yeeeeah if you're looking for deep consistent lore in Mario, you've kinda missed the point.