Bruce so how it works is you preference RAAF pilot at your YOU Session or over the phone and they'll test you on that specific job for your Assessment Day. The additional testing is the same regardless of service.
Once you get to Flight Screening they will explain to you how to change your preferences over the two week course which you can do at any time and as Khargify said, you will be asked in the OSB at the start what you want your preferences to be.
The reason for this is that the Army and Navy give "single service briefs" while you're there to try and convince you to preference them as well, and you can change it at "the window" at any time.