Larry said:

Our puzzle was glued to a lightweight paper backing as each section was completed.

After the full puzzle was assembled, the design surface of the puzzle was coated with a puzzle glue/fixative and allowed to dry.  After the glue dryed, the puzzle was "warped", although the design surface is unharmed.  We are now attempting to determine the best method to flatten the puzzle, in its preparation for mounting, framing and hanging.

Advice is solicited from all who have been down this road before, and would be much appreciated.

Larry