I think the only possible issue here is that the OB booking might be treated as a travel agent booking, as it's not booked through the normal BA.com route. Just guessing here, of course. The big downside to OB upgrades now is the need to book a fairly flexible underlying fare in all cabins - WT+ needs E class, for example, and J needs D, I think. A lot more expensive than the fares needed to MFU (or whatever it's called these days).