I can only speak for the USAF Reserve. Once selected for a UPT slot, that is a railroad to OCS, then UPT, then follow-on training. The rules were, 6 years old now, if one quit or were washed out, you were considered "excess" to the unit assigned, no trained specialty, and discharged. That is what happened to my one "wash out". In today's times of cutbacks and excess manning, I don't see the AFR forcing one to be trained in a career field they weren't interested in and won't stay in.

The flying position is guaranteed, your graduation is not. The funny thing is, you will go for the flying, you might stay for a dozen other reasons or careers. My first UPT pilot is now a squadron commander, having been a Chief of Standards and a Flying Safety Officer. He also is a AA B757 FO.

But, ask the specific question and the potential outcomes.