I must say that this would seem odd to me as well. those fares are negotiated by airlines and it would seem bizarre and somewhat unlikely that one would like to retrospectively reprice.

I suspect that the start of that "rumour" on this thread relates to what happened in the US between AA and DL but the situation was quite different: AA chronically reroutes many more pax on DL than the other way round and AA refused to be part of an overall agreement between airlines, so it was not a case of retrospectively changing rules but rather of AA refusing to sign an agreement all other major airlines subscribed to, and not a case of a one off imbalance but of a persistent inequality between passengers transferred in and out. Between AF and BA, there is likely no such chronic imbalance and cross-airline reroutes are quite a lot rarer than in the US anyway, so personally I am not convinced that this is a likely story either.