you have to unzip the whole project, look for a file called project.c3proj and open that in a text editor, and on the second line of the file you will see:
"savedWithRelease": 16402, (or whatever version you saved it with)
change the number to the version you want to open it with... for example:
"savedWithRelease": 16300,
save the file, re-zip the whole thing (and change the extension to .c3p)
NOTE!!!! this will only work if you have not started using new features not found in the version you are going back to... so make sure you keep lots of backups! and use at your own risk :)