Yeah, they stick pretty damn well... Can't help you on that one, you might have to get new ones to switch. You could maybe look into getting heat cable or heat tape, in the long run it'll be cheaper than UTH's, or you could get them at Reptile Basics, I hear they're half the price of pet stores.
Trick: Get some aluminum tape from the hardware store, and cover the sticky side of the UTH with the tape, shiny side out, so that you can tape the heater on with a couple pieces of aluminum tape on each edge of the heater, and then un-tape them when you need to replace them.