By way of example from experience, Kenya Airways offered 2 x 23kg bags in economy on my NBO-LHR flight. When their flight was cancelled I was provisionally rebooked onto Lufthansa and luggage was only 1 x 23kg. Lufthansa's policy would have prevailed and I didn't want the hassle of trying to reclaim the excess charges from Kenya Airways so asked to be rebooked onto BA which they happily processed and received my full BA benefits.