I moved my sister-in-law's HP laptop from Vista to Win7Pro about 18 months ago. She had been given a Win7 OEM copy from NewEgg as a gift.
The only difficulty that I ran into was the Win 7 install CD was 64 bit & the laptop was not. I downloaded a 32 bit iso file from Windows & made a install CD from that. The install key does not care if it is 64 or 32 bit OS.
The MS Office software loaded on the new os without any issues and the backups of all the data for Office re-loaded and ran without any issue.
I expected to run into driver issues by not using a HP install, but all the video, USB ports, and wifi all worked without any issues.
PLUS she got a computer without all the dozens of "SPECIAL OFFER" software programs that come pre-installed.