Hi!
It is possible to transfer across services. It all depends on the situation that your unit and you are in.
I am going from AF to Army Guard. In my unit, they only will take Huey or Blackhawk qualified pilots. I was Huey, so I'm in. If I was in the Army and only qualified in OH-58s or Apaches, they wouldn't take me, for instance. Check with each unit, what they fly, and what requirements they need. It could be easier to get into an AF unit, for example, than into a navy one. I know a number of Army helo guys who got AF fixed-wing slots.
It may take quite a bit more paperwork when switching services. I've been out of the military 11 years, and am changing services, so the paperwork's a bitch!
Cliff
GB,WI