Maybe I'm misunderstanding, but i'll recap:
BCM450 Base Function Tray (BFT) does not have a separate Media Services Card (MSC) - it is "on board" on the BFT.
So when I refer to the release of the BFT of a BCM450 the system ID is tied to that.
When I look up the System ID of my "defective" BCM450 BFT on KRS, Avaya shows it as a Release 5.
When I look up the System ID of my "replacement" BFT on KRS, Avaya shows it as a Release 6.
I used KRS to "transfer the keycodes" from the "defective unit" to "replacement" unit, there was no previous "upgrade" keycode, nor was I told the transfer was incompatible due to the different Release numbers.
However the transfer was successful, I double checked now and the replacement still shows Release 6.
So I had no problems restoring my "release 5" backup to the "release 6" replacement BFT.