I'll step out of this now, but seriously, when it comes to giving advice about anything licensed and copyrighted, I urge you to verify with an official "published" source first to make sure it says it can legally be done. The Retail license says it can. The OEM does not.
Nor does it say it cannot, the door is still open. In the court of law, I believe that would be called reasonable doubt (a gray area where anything can go).
If you don't feel comfortable giving advice about switching bit versions with an OEM License, that is your business. If you can't show where Microsoft expressly states OEM License does not support both bit versions, don't contradict others when they say otherwise.
This whole concept reminds me of the time I mentioned a few details about Technet Subscription. In the end I grew tired of bickering and decided if Microsoft really cared they would clear up the misconception by publishing much clearer details of what End Users could do and what they couldn't do. If you want to disagree with someone in a thread that is fine by me but don't go so far as to tell them they are possibly wrong unless you can prove it. The topic of the Technet Subscription, I tried proving a few details and apparently failed which is what would happen here if you tried proving the OEM License is bit specific.