I cleaned recently one of my PCs that had some mixed things inside the registry after trying several MJ and MC versions.
At first I used the MC11 and MC10 uninstallers. Then I removed the remaining JRiver associated folders in the "Program Files" and "Documents and Settings" folders (Hidden files display must be enabled first). At last I cleaned the registry. I removed everything under JRiver in HKEY_CURRENT_USER\Software and HKEY_LOCAL_MACHINE\Software. (If you have a multi-user PC you may want to check the HKEY_USERS part too.)
Then I installed MC11 back again. (You need to restore your license after cleaning the registry.)
I made a library backup before the cleaning, so it was easy to restore the database after the clean install of MC11. Only the user defined options were lost. It could be a good idea to make notes about the current options.
At first you could uninstall MC11. That should remove a part of the mess. Then you could install and uninstall MC10 once again. That should remove the MC10 part. After that you could do the folder and registry cleaning steps (if there is anything left).
Remember to copy the library backup file, all media files (if there are any) and your cover art folder to some safe place outside the MC folders.
EDIT
In case you like to buy an upgrade license, you should buy it before uninstalling MC11 and cleaning the registry, because it is not possible after that. (You must reinstall and register your previous version again for getting the upgrade price.)