Opinions vary. Generally speaking, it's poor practice to mix finish chemistries. Usually doing so will either result in the top finish not adhering, or the entire finish reacting in such a way that the only recovery is removal.
Polyurethane is a very durable finish, though. Many solvents that would dissolve or otherwise have a negative impact on most finishes won't affect poly at all. I mean, you can wipe the stuff down with acetone, mineral spirits, naphtha or pure alcohol and it'll just laugh at you. The durability extends to its hardness as well, which means sanding it is just no fun at all. But, that imperviousness means you're probably not going to have chemistry problems, it'll be adhesion problems.
Because of all that, it's difficult to bury decals in it because doing so usually requires that you iteratively sand back the finish over the decal, then reapply a fresh coat. That is, unless you're shooting certain formulations that have a self-leveling quality to them, but those aren't generally used by DIY-types. It requires the proper gear and a high-quality paint environment that most folks just don't have. Lacquer's the way to go.
So, what's a mother to do?
Well, you could strip the poly and start over with lacquer, but I wouldn't recommend it. Lotta work, and a high chance of undesirable side effects.
You could also rough up the poly and put a base coat of lacquer on that, then apply the decal, and follow through with the burial process. Risky, though. You really want a nice, smooth surface to apply the decal to. But, if you were careful, I could see that working.
Another trick is to put a barrier coat of shellac between the poly and your new finish. It acts sorta like a primer, but what you're taking advantage of is shellac's ability to stick to anything, and allow anything to stick to it. Once that's on, you apply your decal, then start the lacquer burial process. Could end up with a mild color change - shellac isn't quite water clear - but you might not notice it with it being just on the headstock face.
I'm actually in the process of burying a headstock decal right now, but I didn't have to deal with the poly thing. I'll have pics up in a few days, but here's where it was several days ago after the first couple coats of lacquer...
You can easily see the edges of the decal, which is why you have to repeatedly sand back, then recoat until everything's level. I'm up to about 8 - 9 coats today and the edges are gone, so I'm probably going to call it good and let it cure for a while so I can finish sand/buff/polish and send it on its merry way.
