https://www.britishairways.com/en-gb...and-conditions

A lot hinges on something called "significant changes", and some is defined (see above) and some is not. Generally speaking any change needs to fit into your plans when you booked. So to give an example, if you booked a weekend break to Rome, BA couldn't realistically move you to midweek or another weekend, that is full refund territory. In reality BA Holidays tend to go above and beyond, in those rare cases that come up. The more awkward ones would be switches from LGW to LHR when pre-booked and inflexible parking is involved, or a timetable change of an hour or two - which is difficult to describe as significant - but could clash with, for example, work rotas.

I would only contact BA a few weeks after any change was confirmed, or immediately if you are within 2 weeks or so of travel. The reason being this forum is littered with examples of people changing flights the moment emails get sent out, and then complaining when the original service is restored a week or two later. As ever, patience is a virtue and at least we have all had plenty of practice of that lately..... Either way it is specifically the BA Holidays team you want, which is an option on the main telephone menu. There is a Duty office telephone number buried in the booking voucher, for anything that happens in the next 48 hours before travel or thereafter.