I do not know if you can upgrade from OEM to RTM (in the way you are thinking, probably not, as the free upgrade will probably be OEM also)
But if you have an OEM License,,, can you upgrade to a Retail RTM 7 license? I don't know.
I want to say I think you can.
You have a previous windows License, you are still upgrading a previously purchased license of windows.
So, I think you can. Someone better versed in the licensing of windows may want to clarify on this a bit more though.
The caveat to this is, you need Disc's to prove to the Upgrade install that you did have a previous license/install of windows. I do not know if you will need the actual previous license, or just a disk. As most OEM systems do not come with disks, they must be made after initial setup through whatever means that company setup for (Dell, HP etc.) to create the restore disks. Most people don't make the disks and end up having to order them from the company (Dell, HP etc.), which in some cases they stop producing, sell out and have none available, so the upgrade purchase could be pointless for some. There are ways around that, but I will not discuss them here.
My wording below,,, was slightly off target of the OP initial question.....
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When you purchase OEM, all following FREE upgrade will also be OEM.
To clarify what others have said.....
OEM and FREE upgrade to OEM, are all OEM software.
Once Activated on a PC it is tied to that PC till it (the Motherboard) dies.
At which point, the license and the PC go in the trash together.
The Disk itself (technically this is a gray area) can still be used. It's the license type that matters most.
An OEM disk should not work with Retail Licenses and vise versa.
The System Builders Kit that can be purchased, is (as far as I know) supposed to only be sold with some (at least one) piece of hardware. In other words, they are supposed to be for actual system builders and not really sold to general public. But this too is a gray area. And this is due to the OEM nature of the licensing. It is also why you get it cheaper, because it is tied to that one activated system. (for clarification) You can install it on a number of PC's untill one activates it, then it is tied to that PC which activated the copy..
So,,, if you are OK with purchasing OEM licensed windows, Then I say go for it. Yes, you do get it cheaper.
You get it even cheaper when actually buying it with a PC, (dell, HP, etc.)
But if Home Built no warranty system dies, and you build a new PC, you have to purchase it again.
And so then, retail ends up being even cheaper.
Also, you might want to check this......... OEM = 32bit or 64bit ,, one or the other only,, not both.
Retail = Both 32bit and 64bit disks.
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