I'm flying LHR-IAH on a 772 (CO5) on Wednesday on a B fare upgraded with miles to ZE. Got a confirmed seat at purchase a few months ago.

The BF cabin is filling up, as expected on this route. How likely am I to retain that seat in a full cabin, without being downgraded to Y?

I normally do this route in exit-row Y. I'd rather fly UA E+ than be downgraded into CO Y, but I'm guessing that's not an easy option in the event of a downgrade? (Short of buying another ticket.)

Still getting my head around the upgrade mechanics.