FYI, I recently completed a similar upgrade on a machine where someone installed 32 bit XP on a 64 bit machine. Perhaps the following anecdote is useful - or not. You be the judge.
Feeling quite clever, and not wanting to loose the configured XP development machine, I virtualized it using VMware Converter. I then did a fresh Windows 7 install and added VMware Player. I expected the virtualized XP box could be used for on-going development while the Windows 7 development environment was setup catch-as-catch-can. Sadly, Windows Product Activation (WPA) foiled the plan. We were unable to logon to the virtualized XP Box as it was seen as probably pirated software. Perhaps there was some way around the issue but I opted instead to take the hit, and setup the Windows 7 development box.
FYI, although we were unable to log on to the virtualized XP box, I found I could map its drives and gain access to its files. This was somewhat useful as (of course) I found a few things that escaped our backup.
Hope this is of some use. Best of luck with your upgrade.