If just the clear coat is breaking down (a common problem seen with older lacquer clear) I suppose it is possible to sand off all the old clear down to the lacquer color coats, preserving the original lacquer color coats, and then re-clearcoating. You may want to use an adhesion promotor prior to clear. I wouldn't use urethane clear as no manufacturer will warranty the product if it is not applied over their base coat product. I had once, a very long time ago, done a paint job on an old Harley Davidson in acrylic lacquer, two tone paint, with a urethane clear coat. When the bike was put back on the road a short time later, the clear coat peeled off in large sheets. When I called in the manufacturers rep, he walked out of the shop when he learned what I had done.
Don
Don