OK so this is a fudge, it can be done but anything that’s incompatible between the two versions could cause you big problems.
I would export each page as a separate partial project and then for each do the following
open the file in a text editor and look at the first few lines
<?xml version="1.0" encoding="UTF-8"?>
modify the major, minor, rev and build to suite, save and try it.
this is at your own risk. its useful if like me you have various versions out in the field and you forget, spend all day on a page and then realise is a newer version.