Sharing some info which might help someone. My mother has been staying with me in Dominican Republic. She flew into Punta Cana PUJ just before the Dominican government banned all flights from the UK and was booked to return just after the ban finishes on 12th Jan. We think it's likely that the ban will be extended and any flights will probably be cancelled in Jan in any case due to the Tier 4 restrictions. Because of her age (in her 80's) we really wanted a non-stop flight to get her home without a transit stop. BA is sending an empty plane over from ANU on 2nd Jan to return the people who flew out with my mother, but no more flights after that. We decided to try to get her onto the 2nd Jan flight, called the Indian call centre and was told that because her flight hadn't been cancelled on 12th, she would have to pay for the change. HUACA and was told that there may be rebooking guidelines coming out for PUJ, so the very helpful guy I spoke to held a seat for her on the 2nd and told me to call again in 24 hours. Called again yesterday and told that there were no rebooking guidelines but the lady I spoke to said she would "dig deep" into guidelines and try to help. And she then came back and said that she is permitted to make one free of charge change in response to changing government regulations affecting a journey. Not sure if this is common knowledge, so sharing here in case it helps someone.
In summary, she said if government rules affect your trip at either end, you are entitled to one voluntary change free of charge.