Don't how you swung that, but be very careful. Upon starting AD, I was paid as an O-3 with 4 or more years of service. In other words, they were paying me for my HPSP time (When I was first commisioned). Despite all of my inquiries and protests, they continued to overpay me. Well, 9 months later they "finally" discovered their "mistake" and I had to pay the amount back over the next 12 months. It wouldn't be such a big deal, if my wife listened to what I was saying and didn't over-budget while we were getting overpaid.
Back to the original point, your pay scale is figured out by the date you graduated med school (not when you were commisioned or when you did your first ADT in med school). This has been confirmed by multiple individuals at Denver and Randolph AFB that strictly deal with military medical professionals only. However, I do not know how it works for USUHS, Academy grads, ROTC people, or Prior service individuals. I can only speak for HPSP individuals without prior military experience/commitments.