Being certified in all that sort of stuff (unfortunately) the below is a quoted statement directly from several different "Microsoft" certification courses I have had to take.....

"It is better to do a fresh installation than it is to perform an update from one operating system to the next...."

Hope that helps you out.
One of the reasons behind this is that each operating system is built with specific "minimum" set of system requirements. Windows 7 requirements might have been less than 10. There are several other reasons but I'll save you all the technical crap and just say it isn't a good idea.