As Qantas F/As used to do in the 90s and 2000s where they operated the 767(238, 338, 336), 747 (400 in Pacific and Kangaroo route configs, and Classic), A330 (200/300).
Their ability to easily train onto the A380 and then the 787 was taken away from them when QF recruited new staff for those operations and prevented those legacy crew from operating those aircraft. This is an issue of Qantas’ making but they’re expecting the F/As to sacrifice everything in solving it.
Not sure if it is completely accurate to describe the situation of being easily trained on other aircraft 'being taken away from them'. I believe it was actually 'given away' by the FAAA, hence my description of the 'pull the ladder up behind you' industrial strategy of keeping your conditions and allowing new-hires to be employed on lower conditions. That decision was always going to end up with them being outnumbered and marginalised at some point in the future.