As you rightly assumed, you sadly know the answer to that. BA will not let you change the unaffected booking for free when a separate one has been affected. Your options are 1) accept one of the rebookings proposed on the cancelled flight or any other flight on the same itinerary which has availability in your original class of travel if any is better 2) ask for a refund on the booking that was cancelled and buy a new one (maybe OS has times that work better for you?), 3) hang on and hope that the other itinerary gets a cancellation as well and then you can rebook everything in ways that fit (though the bookings will remain separate in case there is any further change), 4) change the unaffected booking to make things work now already but you'll have to pay any change fee and difference in fare.

PS: of all ironies, your handle is proving a bit paradoxical in the context of this specific thread