You can yes but am not to sure if this puts a delay on application, your best off speaking to your advisor about this. What did you apply for and what are you wanting to change to and congrats on your A grade
Depends what you were assessed for at ADSC and what you now want to change to. eg if your initial choices were all Corps and you now want Inf the PSSR (run times etc) are different. Vice versa if it was initially Inf and you now want a Corps they will have looked at different selection criteria. There may be a requirement for you to go back to ADSC but you need to speak to your recruiter. You will probably need to be re-interviewed at least once as they ACIO/AFCO will have to assess your job knowledge etc again for the new choices.