Reporting a success as a follow up to a previous post.

My incoming Avios flight to LHR was delayed, so I would have missed my back to back round trip TP run that was an hour after the scheduled time of arrival.

On the flight, the WiFi is free for MMB, and I used the online portal to change both flights. The portal wouldn't let me rebook onto a back to back itinerary sadly, and said the inbound must be 2 hours after the outbound arrives.

However, when I went to a check in desk at T5, a kind lady was able to change the inbound to the immediate turnaround flight thankfully! It did come up as a "cancelled" flight, but still worked well.