OEM can mean two different things
I'm thinking the upgrade is an OEM version ...
In that once installed in a computer .. It can never be moved to another.
I upgraded my Vista Computer to an OEM XP MCE.
All this means is that it was cheaper and now cannot be moved because it's only licensed on the computer it was originally installed in.
If the upgrade is sent by M$ .. It may not install the OEM supplied applications already in Your computer.