Hmmmm....I've seen the adhesive darken a white acrylic or polycarb sheet like that, just where the lettering is!
if I recall, standard procedure for used faces; if you've taken an accurate survey, then the options are....
1. make a new face, to the correct dimensions...and swap with the old one, thus saving an extra trip, setup, leaving the lamps exposed etc. etc. Show the customer how this saves them money, and they get a face that can now be re-used numerous times.
2. Turn the face upside down, flip back to front, if the retainer permits...to reduce 'readability' of the old image obviously.
3. A quick cut-polish if you have an automotive buffer...removes the part of the ghosting that is oxidation, plasticizer migration, adhesive staining etc......essentially as good as the old face can look!
4. The new graphics should NOT be skinny letters on an expansive blank background ( where the ghosting is most obvious!)
re-design to make the entire face a reverse panel - dark background, with the lettering illuminated...little if any of the old image will be obvious.