I would try polishing any decals that are in an inconspicuous area first (if you can). I've had great success polishing and waxing over Tamiya decals, although it was right after they were applied.
Are the edges of the decal silvering? Are the decals lifting anywhere?
Here is an example of decals that were polished over:
http://www.automotiveforums.com/vbul....php?p=4820433