How likely is it for an AH-64 pilot to transition over to 15C?

Lt, timing, timing, timing is everything in the Aviation community for straight commission or warrants. Typically , Lt's get the fixed wing slot before advanced airframes are selected. We had 2 slots in our class both given to Lt's. My experience is these individuals made their case early on to the chain of command. At this point you might have to show up to your first duty station and then ask your chain of command the steps to put in for the transition. Also, check with MI Aviation Battalions in the field. I know the one at Ft Hood as I worked with them in the past. Do your research on your down time at Mother Rucker and you will be amazed what might happen. God Speed.

Getting a fixed wing slot is pretty hard to do, but not impossible. Your only opportunity to do so will be requesting the Military Intelligence Captains Career Course with a fixed wing transition. If you go to normal Aviation CCC, you will never be a 15C. For us RLO folks that is the ONLY opportunity to get it outside of flight school.

Source: I'm an RC-12X pilot in an MI battalion.

I would suspect that since a lot of the Apache battalions are part of the attack/reconnaissance role, 15C would be a shoe in for you. Best of luck in your endeavors.