I’m not the owner of this trick, but I can confirm this works well.
Before start the installation, just change the two keys in Registry. Its like you are faking the Windows 7 installation that you are using the same version which you are going to install. In my case, I had to change the key to ‘ Windows 7 Professional ’. Make sure you are typing the exact and correct OS version which is written on your installation DVD
After you change the registry key, insert the Windows 7 Professional installation medium and start.
Make sure you have enough of space in ‘System Partition’ (usually C drive). Allocate enough of time since this will copy your personal data first, downgrade the OS and then copy back your data.
I suggest to clean up (move to different location) your personal data (such as Desktop, My Documents..etc) before start the downgrade for saving time.
Because of this method worked great for me and I managed to downgrade Windows 7 Ultimate to Professional, I recommend the original source with more details and screen shots.