I don't know about the OP's flight but we've seen in other threads (BOM) BA treating these changes as cancellations, with a new flight immediately created. That makes sense in the context of EU261: it grants then the 14 days' notice get-out, although I'm not sure it would stand up in court.

Personally I think airlines are entitled to retain flexibility and if they need to change schedules or in this case planes with 14 days notice, it is in line with the aims of the legislation for them not have to pay a penalty/compensation (the 75%); rather they should just have to re-accommodate or re-route the OP as best they can.

I expect the OP might feel aggrieved though as presumably other passengers have been given the new F seats in preference to him. The real kick in the teeth will be if BA want to reprice J at the new redemption rates, as seen in the BOM thread ... I would hope they would manually adjust that, but it does seem to be inline with the legislation: take the later F offer or effectively cancel and rebook in J.

Should I mention a hotel must be provided only if the new flight is "at least the day after" ... ouch!