So you're saying that you do not have an earlier version of Logic on your machine? Check your Applications folder, if you have more than one Logic icon you have an earlier version on your drive. In which case you can just drag the later version(s) into the trash. Then rename the earlier version simply "Logic Pro".
Also, do not be afraid hulk, if you back up all of your system including all files before you attempt this downgrade you will not lose anything.
Let me say that again a little louder... BACK UP!
Lose project files? Changing the application version will not delete your project files.
Lose custom patches, etc.? Not sure. I haven't personally experienced your situation so I can't comment on that.
Nosce te ipsum