Djmp92,
Its important to do a hell of a lot of research into each service and figure out exactly what you want fly for a living if given the opportunity.
What you put down on that sheet is just to give the defence interviewer at assessment day a service to primarily focus on during questioning.
You are able to change where you rank each preference all the way up until your sitting in front of the board. Therefore, you can change without incident even when your on FSP.
Whilst on FSP you have the opportunity to rank each service from 1-5. If you put a service as 1-3 you will NOT get an offer - even if you are an ACE. 4-5 you will get a look in.
The way the board explained it was that your overall score on FSP is combined with your perfs for an individual service. That combined score is then put into the pool for distribution.
So you do need to ask yourself. Do I want to fly for the ADF or just fly for an individual service.
Sorry for the long response. Hope it helps.
Only a couple of courses left for the year. Coming up to crunch time now.