In general, if you are changing only the processor and keeping the same board, OS re-installation is note required. No update or whatever is required. However if you are changing the motherboard and more importantly to a different architecture, clean operating system installation is required.
You may use the OS repair features to try to fix the existing operating system to get it to adapt to the new platform, it may work fine for some time, but at some stage you will definitely experience system instability and many issues will start happening.
Hence, broadly saying, in some situation it may work, but in most situations the system may experience weird issues within the system operation.
Best recommendation when changing motherboard, is to re-install the operating system with a clean install to ensure system will work fine without any issues.
Hope this will be helpful to you.
Support at Laplink is saying that I can make a copy of the existing drive, upgrade the MB and Processor, do a clean install of W7 and then use PC Mover Pro to reinstall most of my existing software. I assume it would be better to do a fresh install of programs such as Photoshop which make use of the 64bt architecture. But I am also assuming that some less precessor intensive program such as Quicken or Firefox will run fine without reinstalling....hopefully. I do10 different tasks from, accounting to video editing on this box.
You can also keep your old windows XP as it is and install 7 on a different hard drive, and that way you'll have a clean install of windows 7 and your old windows XP which should still run fine (though you changed your system) and that you will be able to use would you have a program that doesn't work with windows seven - it happens with some old ones.
Anyway, everything that has already been said here is just fine.
Oh, one last thing : you can virtualize windows XP with 7 professional (which means you can run windows XP inside 7) to help you run older programs. It works better than with the compatibility mode. thats called the "windows xp mode", more info there : http://www.microsoft.com/windows/virtual-pc/download.aspx
and with Virtualbox you are able to run your old windows XP, as it was, inside windows seven, from another hard drive :