Cancelled flight – how long to obtain a refund?

[Cal]

Hi
I have a BA short haul cash booking for November. I’ve just had an email to say one of the flights has been cancelled and we’ve been rebooked on another one (15 mins later). Whilst at the moment we do intend to go and the new flight is fine, can anyone tell me how long the option to obtain a refund exists if I do not actively accept the new flights?

As it’s still 4 months off, I have realised that having the option to get a refund on an otherwise non-refundable ticket could potentially be useful if life/plans change nearer the time.

If I do not actively accept the new flights, am I still booked on them unless I request the refund (don’t want to risk not being able to travel on this flight if all ok, as the cost of cash flights has gone up quite a bit!).

Thank you