Can work... or can be tricky.
I built a new machine for my bro - to replace his which was getting flakey... in the end I gave him machine and just put his old HD in (win XP)... had to reauthnticate windows and it did some driver updates - but it worked fine.
Put the HD from the new machine into his old machine (which was a fresh XP build) .... did not work at all. To be honest - I didnt spend any time on it -I just resinatlled Windows fresh again and it was fine.
So you may be lucky... or you may be unlucky. I guess if you new Mobo is close (manufacturer... chipset similar etc) you stand more chance it working.