You are correct, it's not absolute. I went through IFF (late 2000s) with a guy that originally tracked T-1s and went on to a Guard KC-135 squadron. A few years later he somehow got an F-16 unit to hire him. He was sent to Randolph and got a 20ish ride program in the T-38 and then sent to IFF. Unfortunately for him, he washed out of the B-course. Nice guy. Last I heard he was back in the tanker and flying for a major airline.

I would imagine this is extremely rare and most likely would only ever happen in the Guard. Although, I don't they'll ever try that again.