It may be good idea in future to add compatibility options to Save dialog.
Example: CS5 has option to save project in regular format or CS4 compatible.
Photoshop and Illustrator have had similar over the years. Now, a PrPro Project is far more involved than a simple PSD, or AI image file, so much more would have to be done to make that happen.
Historically Pr/PrPro has never had backward compatibility.
Maybe file a Feature Request.