wow that’s really cool!! i kind of wanted to swap their main traits but not everything literally, ex. papyrus is more into practical jokes than puns and sans likes riddles instead of puzzles. i also thought it would be fun if sans was into mexican food instead of pastas ex. he enjoys making taco’s/burrito’s.
as for the swaps here’s the ones i had in mind/that have been suggested for now:
pap swapped with sans (obviously lmao)

undyne (shes now the royal scientist who enjoys cartoons, she no longer had the eyepatch)
swapped with
alphys (the fierce head of the royal guard that wields a huge battle axe and has dino skull themed armor, instead of the eyepatch she has a claw scars vertically across her left eye)
toriel (the ‘ruthless’ leader of the underground screwed up and killed the humans)
swapped with
asgore (escaped former king tend to fallen humans makes them cakes, was disgusted by toriels actions and left her)
mettaton (shy ghost that hides in his house dreaming of becoming a famous star one day/ tends to his diaries ussually)
swapped with
napstablook (number 1 radio DJ in the underground, his body designed by undyne has retractible turntables to mix some sweet jams)
i dont know about the other minor characters but this is it for now!! this rant has been long enough… *cough*