Really, probably the best way to put it: Once you are at level 100, there are very few things you can do to change your pokemon. The ones that come to mind are: use evolution stones, feed it pokeblocks (if it hasn't had 12 already), feed it vitamins (if it's not fully EV-trained), and feed it berries that will lower it's EVs (not advisable because you CANNOT fully EV train a lv 100 pokemon... unless you do a 100/100/100/100/100/10 pathetic vitamin spread). Correct me if I missed something.