In normal times you should get a notification( normally via email if you've given BA one) nearer the time advising of the change. Waiting for this will probably be your best bet as whilst we can all agree now that the change has happened already you do risk a wasted phone call giving you the response of "we may change it back and therefore are unwilling to compensate at this point" @
corporate wage slave etal may be able to advise better/ drill into the minutiae better than I but when an IIROP occurs in advance of the flight ( normally dialled in 'for sure' at +72 hrs) communications
should occur giving you your options.
I will leave the 'current F Vs new J' argument aside and would suggest exercising your rights( to claim comp) over personal choice discussions regarding the two cabins.