I'm no expert, and my time in ROTC was long ago. But here's the way I remember things working.
If you get picked up by an ANG or Reserve unit while still in ROTC, then it's up to ROTC as to whether or not they want to let you go.
If you get picked up by an ANG or Reserve unit after commissioning, then it's up to AFPC if they'll let you go.
The process to get interviewed and hired in the ANG and Reserve is kinda lengthy. If you're an AS400, then I'm guessing you have somewhere between 6-18 months before you commission (depending on if you're going to do a 5th year or not). 6 months might be kinda tight, but do able, if you start now and the interview dates line up.
Back in the early 2000's, Active Duty was hard-up for people, and they weren't allowing anyone out to pursue ANG or AFR opportunities. Now a days, it seems like they're more willing to let people go.
Probably something you could schedule an appointment with your Colonel and ask about.