Well, the Win 7 Professional upgrade comes with both versions and you will have to do a clean upgrade, but you still need another key. You can get those from the Microsoft site so you might check. If you decided to get the Win 7 Home Premium version, you could do an in place upgrade which would allow you to keep you old files.
Remember, you need an installed, activated version of the qualifying OS, Vista in your case. You will not be asked for a previous key during the process and the old OS will be deactivated so you can't activate it again, or so I hear.