Since decal adhesive is water soluble, soaking the body long enough might just work to get those old decals off, but once they're off they'll be almost impossible to use. You'll most likely tear a couple, they'll curl up when they dry and will be brittle, you'll have no easy way to reapply them, etc.
A much better way to go is just buy new decals. Here's one source:
http://www.hometown.aol.com/vintagedecals/monogram.html