You want to be VERY careful trying something like that.
OEM licenses are locked to the first motherboard they activate on - and cannot normally be moved. (MS appears to be enforcing this again now, after a few years fairly lax control)
With Windows 7, once you go out of grace period, the penalties are relatively minor - mostly consisting of black background and the on-screen notification.
The best thing to do is to ensure that you are still using the OEM_SLP Key (run MGADiag to check) with which it came pre-installed and just let it go out of grace, and do NOT attempt anything fancy in terms of activating or attempting bypass activation.
So long as it's no more than 30 days, even MSE will continue working - after that, it will shut down (with warnings)
Once it's back in the proper machine, you may need to simply validate at www.microsoft.com/genuine/validate
or (at worst) apply the COA Key, to get back to a genuine status.
If the COA Key is in place already for whatever reason, then so long as you don't allow the system to activate, you should be OK (but if the motherboard has been replaced then all bets are off, although it should be a case of using telephone activation rather than anything else).