Use a heat gun or hair dryer and strong fishing line. Warm the emblems and then use the fishing line to saw through the 3M adhesive. Use something like Goo Gone to remove the rest of the adhesive on the vehicle.
Edit: when replacing the emblem, if you look around, you can find the gray 3M tape/adhesive that you removed. It's skinnier then the white 3M tape/adhesive normally found. I think O'Rielly's may carry it.
Good idea to take measurements where the emblems were, like Mytoy4 suggested.