A concerned reader writes:
The other day you said that once you open and edit a Pages '09 file (or another kind of iWork '09 document) that file becomes irrevocably linked to the newest version of the application. Is that really true?
No. As has been pointed out to me by a reliable source, there is a way back. I've updated that article to explain how, but it's an important enough bit of information that I want to give it its due.
When you open a document created with any iWork '09 application in its iWork '13 counterpart and then make a change, its file format is altered to a form incompatible with the previous version of the application. In this format the file can't be opened in the old Pages, Numbers, or Keynote application.
However, you can restore the original version by choosing File > Revert To > Browse All Versions. In the Time Machine-like interface that appears, browse back to the first version of the file and restore it. This will replace the version you've edited in the new iWork application with a copy that can be opened with the old iWork '09 application.
Of course this means that you've lost any changes you've made in the new application. But it also means that this document once again becomes available to edit in the version of the iWork application you prefer.