Prior authorization for medication question.?

I have been diagnosed with ADD and have been paying out of pocket for my medications until now. My psychiatrist finally got my prior authorization accepted by my insurance and wrote me a script for adderall xr 15 mg (qty 15) to find out the right dosage for me. If I end up needing a different dosage, will I need a new prior authorization. Also I have a prescription for 45 15mg capsules that my other doctor had given me about a month ago when the prior authorization was declined. Can I fill this now even though it's a different qty? Thanks!