There is a way. See Downgrade: Change Windows 7 Ultimate to Premium or Professional. I successfully downgraded from an unregistered Ultimate to a registered Professional edition using a universal installation DVD. You edit two registry keys to match the edition you want, then use the DVD to "upgrade", without booting from the DVD.
Here are the two keys to modify:
Here are the two keys to modify:
\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\EditionID
: change fromUltimate
toProfessional
orHOMEPREMIUM
\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProductName
: change fromWindows 7 Ultimate
toWindows 7 Professional
orWindows 7 HOMEPREMIUM