If there's an equipment change and you do not like your new seats and you cannot reserve seats on the new aircraft that you do like, then I think you're entitled to a refund of the fee paid. So other than the hassle of chasing for the refund, you shouldn't be at risk on this account.

At any rate, I would expect that by the end of February you should be able to be pretty confident that there will be no further systemic changes of equipment. There may be ad hoc equipment changes, perhaps to cover for maintenance that hasn't yet been firmly scheduled, but you can never predict that sort of change and it can happen right up to the day of departure - so that's not worth worrying about.

The longer you leave it, the more likely it is that the seats you want will have been taken by someone else. However, if you have a good reason for delaying the seat selection, then you can always periodically check the seat map either via ba.com or ExpertFlyer.