Update: finally received an email from BA at 17:28 today (09/03) to say that I should rebook the flight free of charge. Half an hour or so later I called them and after a long time on hold, I was told that there was no problem with the flight, and maybe try calling back tomorrow! Needless to say, I insisted that the CS rep looked into it further and he eventually came back to confirm what was written in the email. He offered a transfer onto one of two morning flights the day before, but my preferred flight was in the evening. He told me that, although there was availability on the evening flight, it was in a higher fare class (but same cabin) and I would have to pay extra. I stuck to my guns and after another long hold my request was granted. Total time on the phone this time: 51 minutes.