However you *Could* be lucky -- I've actually changed a MOBO and my version of Windows 7 didn't actually ask for a re-activation (Windows 7 Ultimate X-64 Full retail).
A Full retail version if it wants re-activation will be no problem --just phone the MS activation nr in your area and they will always re-activate unless it's a pirate copy or one where the number has been blocked for example technet copies going out into the public domain.
OEM versions have a certain latitude into what changes can be made -- and although in theory they are tied to the mobo you might just be lucky.
Probably you'll have to get a new copy however -- that's why whenever I buy a machine even if it come with a pre-installed OS I always try and haggle for at least a Home Premium retail version to be thrown in with the machine which I can upgrade later via anytime upgrade.
Stores need to get rid of some of their stock so asking for this cost them very little and you get rid of all the pre-installed crapware too when you install the proper Windows 7 version.