Opps, yeah, guess I should have completed my thought. Boil to get the sticker off, and it also helps that it's warm when applying the new one.
Like I said in the first post though--be very careful when you boil the center caps. I warped one of mine b/c I wasn't paying attention to how long it was in there. I barely had to use any goo-gone to remove left over residue using this method.