This is one of the reasons that I don’t like to put CTP software (or Beta software) onto machines that I can’t take the time to wipe and rebuild. Buck Woody wrote a note about upgrading your CTP, and the short answer in his blog is:

NO

You can’t upgrade a CTP and it’s not supported. Well, maybe you can upgrade a CTP, but it’s a bad idea. There could be settings, registry entries, etc. that get stuck from the CTP into the RTM, and it might bite you later.

I don’t think it’s worth the risk. Honestly SQL Server is easy to detach dbs, script logins, uninstall, reinstall, fix it up. It’s a simple process, even if you have jobs, so I wouldn’t complain about it, wouldn’t chance it, and just rebuild the SQL instance.

Here’s what I’d do if you don’t want to rebuild Windows

1. Perform a physical to virtual migration using Hyper-V. This will give you a backup.
2. Script out logins, jobs, save packages on the file system. If you have linked servers, do those as well. I might do a complete script of the server if you can.
3. Detach the user databases.
4. Delete tempdb (edited thanks to Glenn's comment below)
5. Uninstall SQL Server 2008 R2 CTP (or whatever version). Make sure you get all Control Panel SQL Server entries.
6. Delete the registry keys that might exist.
7. If the user databases are in default locations, move them. Delete the folders from the file system.
8. Reboot
9. Reinstall SQL Server
10. Attach databases.
11. Run scripts to add logins, jobs, etc. Fix orphaned users.
12. Test, test, test,

Easy-peasy