Old peeling decals

Decal solvent.  I know some folks who have used future, which gets a glossy surface under the decal as well as resetting, but I prefer solvent.  You may still have silvering, but if you were satisfied with the model before, just the decal solvent may return it to original condition.  Just be extremely careful after you put the solvent in place, and try to press it down (and any repositioning) immediately.  Once the decal solvent works for even a couple of minutes the decal will fall to pieces if you touch it.