I just returned from the 7-night Western (3/12 - 3/19) and was assigned PLA rotation. The first day on-board I went to where you make Palo reservations and requested to change my rotation to APL and they did. Which was great because we had the greatest table mates!
Our ship was full but they still were able to change my rotation. But we did have the late seating and I am sure it is easier to change on a late seating than an earlier seating.
Pirates night is the night you are in Cozumel on the western (Wed). With the APL you will be in Parrots Cay for Pirates night. And you will be in Lumieres, the most formal of the 3 restaurants, on formal night and semi-formal night.