You can "upgrade" from the RP to the RTM. You'll lose all your files, but you don't have to go back to W7 again.
Are you sure you will lose your files? Normally when you perform an upgrade to a different version of OS (certainly with past windows) it places the files in windows.old
It's still not ideal because you have to reinstall programs but at least the docs/files can be copied back.
Can anyone clarify if what I described happens when upgrading to windows 8?