There is no need to upgrade to Ultimate which requires changing edition ID to run a second in-place downgrade to Premium. You can instead upgrade to 7 Home Premium and then insert the Pro Product Key into Anytime Upgrade which unlocks the Pro features in 15 minutes.
Back up all files and Save externally a Windows 7 backup image so you can start over if necessary:
Backup Complete Computer - Create an Image Backup - Windows 7 Forums
Unlock all versions in the Windows 7 installer by extracting the ISO using ImgBurn, then run
Ei.cfg Removal Utility in Elevated Command Prompt. Burn to DVD using Imgburn at 4x speed with verify.
Now from the Vista Home Premium desktop run an in-place upgrade to Windows 7 Home Premium. Select to update installer and remain connected to the internet, leave Product Key blank during install.
Then type Anytime in Windows 7 Start Search box, insert Professional Product key to unlock Pro Features in Home Premium and activate Windows 7.
**The are paid licenses involving an higher version Upgrade so I'm offering the workaround as a convenience.