What do you mean by "in the same class?"

Do you mean "cabin", which would be strange, as BA is selling ATL-LHR-GIB in both economy and business cabins to arrive on Dec 16th?

Or do you mean "fare bucket" (i.e., the price category), which doesn't, or at least, shouldn't matter for involuntary changes?

I wonder if you were booked in premium economy, which requires one of the lower business class buckets to be available for an onward connection in Club Europe, but which will default to a higher category economy fare if none is available. Even so, that seems like a BA problem, even if it can't be sorted easily on the phone.