Its common knowledge that TA bookings cannot be changed on ba.com, but I'm in the middle of an EDI-LHR-SFO/SFO-LHR-EDI trip in WT+ and while in MMB, I noticed that both the Change my Flight and Upgrade My Flight with Money links were active.

I've gone through both and, in both cases, ba.com has let me choose different flights/upgraded class and I got as far as the payment page.

I don't want to actually change my flight and I'm not paying nearly £3K to upgrade, so can't say for sure whether either would be allowed, but could it be the case that BA now "control" the booking since I've flown the outbound leg and would let me change the flight or upgrade?

The Upgrade with Miles link is still greyed-out, but there's no MFU availability so I can't say whether I'd get as far with that.