So for clarification the policy applies for BA and IB. it states
Rebook into the same class as original or if not available, the lowest class available in the same cabin
So when it comes to rebooking on BA flights it’s possibly if there is space in the cabin regardless of selling class booked on originally and available.
However, the caveat is if rebooking and you were in U, P, G, and X, you can only rebook on to IB if at least one of those four is available.