My DD is in y10. Some subjects will be more affected by board changes than others.
eg English - could be critical, especially if controlled assessments have already been done, though I don't know whether these can be 'transferred' between schools or not.
Maths - I would not expect to be much affected
Science - there will likely be a lot of common core but by changing she may have missed odd bits - should be rectifiable at this stage by going through revision guide and asking the teacher.
History - will be affected not so much by board than by syllabus the school has picked - eg American West & Medicine Through Time OR 20th Century.
Languages - I get the impression there is quite a lot of overlap in the topics covered (and therefore specific vocab). Obviously general grammar and common words would be the same.
Geography - no idea, DD dropped it like a stone.
What might be more of an issue is whether your chosen school blocks options in the same way, and therefore whether your DD can continue with all her subjects, or whether some now clash. She may need to be willing to drop one option, which then of course frees up time to catch up missed topics if she has had to change syllabus.
Additionally of course schools aren't obliged to teach in the same order, so even sticking to AQA she might cover some things twice and miss other bits out.
tbh I would be very tempted to pick an area with loads of good secondaries (eg Hampshire - Winchester/Romsey/Chandler Ford area) and locate there. Then find the best fit / most accommodating school with a space).