Hi all
Just to clarify a few things about MFU - I know it can be confusing.
As you know, you can upgrade bookings made in certain classes using BA Miles. The eligible booking classes are H class and above (Y,B, H in economy, W,E,T in World Traveller Plus, and J,C,D,R,I in Club World).
If you would like to make changes to a booking you have upgraded using your miles before you have travelled on the first sector of the whole journey, you can do so PROVIDED the original fare paid permits changes, and if changes are permitted then the fare rules of that part of the ticket apply AND there is redemption class availability on the flight you want to change to. If a a fare rule change fee applies then this will need to be paid in addition to any applicable booking/service fees. I think this is where the confusion lies because some bookings made in B and H class and maybe even Y permit changes on collection of an additional fee, often the £50 referred to here.
If you make a change to a booking you have upgraded using BA Miles AFTER travel has commenced then you can do so provided the original fare paid permits changes and any additional cost is collected. However, you will need to travel in the original class booked as standard redemption rules also apply and you cannot make changes once your journey has commenced on a redemption ticket.
I hope this helps to clarify things but if not please let me know and I will do my best to make things clearer.
Best regards
Nicci