Unfortunately, MS wants you to use Anytime to Upgrade between Windows 7 versions. You are left to clean install if you have another retail version.
However, there is a workaround which is achieved by adapting this popular fix for changing between the Ultimate RC and any other version: Upgrade the Windows 7 RC to any retail version Icrontic Tech
The same principle applies: you carefully
change the reg keys to trick installer into thinking you are upgrading over the same version, which is a Repair Install in Windows 7 and therefore allowed.
It has worked every time tried so far, for many different combinations Starter>Premium, Ultimate>Pro, Premium>Ultimate, etc. All users report good performance.