"Preinstalled equals OEM."
Not quite as black or white as that nowadays. It depends on what the keycard you use to activate it says - and some of the "higher" versions of Office don't come as OEM: only retail or VLK - but the key still works with the preload created by the Office preinstallation kit.
Even if what you have IS an OEM key, then M$ typically refresh the registration database every 120 days anyway, so theres a good chance you can reactivate it online, And even if you can't do it online, then use the phone option and answer the questions "correctly". I face this problem all the time when rebuilding machines with blown motherboards.