For, example, when the crew turn up at dispatch, they find the A350 they were expecting to fly has been substituted for an A320 due to low pax load. Happens all the time in other Airbus equipped airlines.
A little confused... In a different world maybe.
Mixed fleet flying is certainly not that simple!
A330/340 (was done?), A330/350, out of the loop a little but we all know the reality.