Wow, I've actually seen 2 people change careers over at an AFB I just rotated at.
1 was an Internal Medicine R1, decided it wasn't his field and wants to go into Path instead. I talked to him and he is becoming a flight surgeon before applying for pathology GME slot next year or the year after.
The other was also an R1, decided FP wasn't for him and actually wants to specialize in Pulm/CC. He is opting to do 3 or 4 years as a flight doc or GMO and reapply to civie residency when he gets out. He also had an option to head over to another AFB as an R2 in internal medicine, but last we spoke, it sounded like he was opting for the flight doc tour.
Good luck!!