I think it works like this:
Windows 7 OEM is tied to the motherboard so it'll want a new copy of Windows 7 if you get a new motherboard.
Windows 7 Retail is fine to upgrade the motherboard - in fact I did exactly that and it was fine until my system got all glitchy.
There is no difference between OEM and retail versions now mate.
Basically M$ and tons of other companies were taken to court and lost.
The reason?
Well, with XP and Vista there were tons of different versions. M$ did it simply for fun. So, when people went out and bought say, a laptop with a GENUINE WINDOWS OMFG and then had a problem with it they realised that the f***ers hadn't like, given them an install DVD. So they phone for one but they won't give them one. So now they got a COA stuck to their laptop/PC but no OS to use it with.
So what happened then was they went off in search of one only to find that there were like, ten million different versions and their serial wouldn't work unless they found the exact specific one. So for example if you had say, an Alienware COA in your desktop and lost your DVD you would then need that EXACT version of that EXACT type of Windows or it would simply spit the serial and tell you to f**k off.
With 7 the law was changed dramatically. ANY version of 7 that corresponds to your serial (say, Win 7 pro) will take your serial and work. There is no corporate, there is no OEM and there are no other versions. I know this first hand as my license was given to me by a graphic design company who I freelance for. However, the guy did not have a DVD to send me (as he only got one) so I had to go find my own. I got it from Technet and it worked first time.
See also - the browser selection. That wasn't done for fun that was a legal requirement with ALL versions of Windows.
Edit. Keys however ARE different. A retail key can, IIRC be used on two or three computers.