Even with OEM, you might not need to re-install. I've done some changes with motherboards and CPUs on OEM machines without needing to re-install (even though it is supposed to be tied to the motherboard). The CPUs shouldn't be an issue for that, the motherboard is usually the deciding factor. You can try exchanging the board (regardless of whether you have OEM or retail Windows) and see if it works. If not, then you can also try putting in the old board and installing drivers for the new one, shutting down, and try upgrading the board again. If it works, great, if not, then a re-install is probably necessary.
However, I'd recommend a re-install even if it works without one. It's good to re-install every few years to clear out any junk in the registry and such that can slow the computer down or cause increased instability over time. Exchanging motherboards is a great reason to re-install.