I'm flying to BCN with a group of friends in late July to/from LHR. We booked our seats in the January sale and a few weeks later our LHR-BCN flight was cancelled and BA automatically moved us onto the flight that departs before our cancelled one. I got an email telling me this and a link to MMB which allowed me to confirm the change or the ability to select a different flight at no extra charge. The earlier flight wasn't acceptable to us so I tried to change it online to no avail.

I rang BA and the agent couldn't have been more helpful. She moved us on to the later flight, reminded me that I could have a full refund instead (no thanks) and then proactively told me that as there had been a cancellation, I could change our inbound flight too for free. I moved our outbound flight to the one later than we had booked. I hadn't originally selected that flight as it was considerably more expensive but that didn't matter.

So an automatic change and then an offer to move the inbound flight without charge or fare difference.

I'd suggest ringing back and seeing what another agent says.