If .idml export doesn't work (and if you haven't editied it in CS6, I'd actually recommend exporting from the original as .inx from CS4), try moving the pages to a new file (create a blank file and rename the master page to something that youhaven't used in the problem file, then import the Basic Paragraph style. Select all the pages intheoriginal file in the Pages panel, choose Move Pages, and pick the new file. Insert BEFORE page 1 to avoide swapping sides on a spread. Delete the last page which is the old page 1).
If that doesn't work, it's time to copy/paste to a new file.