It is worth taking a screen shot of a flight where you were in business/first for use later on when travelling in Y.

In Dubai, for both for customs and transit a quick glance of what they think to be your boarding pass with a blue background and business class showing at the top and will be enough for them to allow you through.

This can also be used in BKK, where sometimes, there are no fast track passes on board and so a member of staff waits to hand them out as you exit the gate. Again they do not have time to check everyones boarding pass, they are just looking for the words first/business to handover the fast track card.