Agree with Hoops, you do see people who started out in one service often end up in the Reserves of another, or Air or Army Guard. AD to AD depends on the needs of the service. You also have to keep in mind branch/community career paths - by 5 years and being at O-3, you are expected to have accumulated a certain level of experience and knowledge, usually specific to your service and professional area. The accepting service has to deal with the lack of knowledge expected of an O-3 in their service. The more disparate the professional areas, the more unlikely the leap.
Things I have seen:
- USNA grad went Navy air, Marine Air needed pilots so he switched to get jets, left AD USMC after 12 years and now flies for Air National Guard as an AF officer.
- USNA grad went to med school at USUHS, served as Navy doc, got out, now serves with National Guard as an Army doc
- USMC infantry officer, got out, used GI Bill to go to seminary, came back in as Navy chaplain
- USNA grad didn’t make it through SEAL training due to injury, served as a Surface Warfare Officer, applied to go Army and got in - I just remembered the name of the program,”Blue to Green”