New KB article: Upgrade to Ops Mgr R2 fails with "An older version of Operations Manager 2007 that Setup cannot upgrade has been found on this computer"

When one tries to upgrade OpsMgr SP1 to OpsMgr R2, this error message is to be found in the MOMx.log: ‘An older version of Operations Manager 2007 that Setup cannot upgrade has been found on this computer. Uninstall the older version and run Setup again.’

This KB article describes this issue and how to solve it: KB2004885.