I went from an ASUS M2N68-AM SE2 to an ASUS M4N68T-M V2 with no problems other than switching audio drivers. The M2 was Realteck and the M4 was VIA. Went from DDR2 to DDR3 too with no activation prompt.
Windows 7 Home premium OEM. To be honest its not that big a difference in hardware, both motherboards have the same chip-set, but it was still nice that I didn't have to activate again after the swap.