If you read through Glenn's "
Cleaning and repairing the Shellac clear coat" thread (third one down on the Vintage main page) he speaks about what you need to do to keep from accidentally taking the decals off... which means that he's also alluding to the exact sort of things that
will take the decals off. Fortunately, the shellac was applied on top of the decals, and the decals applied on top of the Japaning (the black paint). One should theoretically be able to remove the top coat shellac and then the decals without damaging the paint underneath. In my mind, the tricky part might be what to use as a protective coating over new decals.