Wecome to the real world of paper conservation.  There are thousands of stories
like this out there.  (Your print could have been mounted down to "drywall" or e
ven mounted  backwards, the error discovered and reversed by the framer, with th
e owners perplexed why it ended up looking foggy and yellowish.) Your example  s
ounds like a standard photographic drymount problem.
 
The problem may or may not be difficult to reverse.  On the one side, the framer
 may have done a bad job and adhesion between the print and the drymount poor.
On the other hand the adhesion may be very good and a mechanical removal not pos
sible (especially if the print is on an easily damaged paper, soft or brittle.)
 This is all speculation without seeing your actual delemma, but one thing is un
contestable -- this is not a job for you to experiment with.  This is a job for
a professional conservator. I say this not just because there may be a very high
 level of manual skills involved but also because of the risks to the art and yo
ur health.  I'll say no more.  Contact the American Institute for Conservation (
202-452-9328) for a referral.