"..With windows 7 OEM the license is tied to the first motherboard it's installed on..."
No. I have done MB swaps before with win 7 OEM.
You may be referring to the OEM restore disk provided by HP, Lenovo, etc. WIN7 OEM itself is not tied at all. If win7 detects enough part changes (CPU+MB+...) that it MAY be running on a new pc it asks to call home or asks you to type in a key you can get over the phone. I've done this too both ways. works fine.