Careful! This is potentially dangerous.
The .caproj project file is essentially a (human readable) XML file. There is a version tag somewhere near the top, which you can edit.
If you saved as a single file (.capx) you have to extract that first, that can be done with any ZIP-program, it's just a ZIP-container with all the project files in it, including the .caproj
That does not always work though. If there was a breaking change to the file format between the versions, it will blow up and be corrupt.