I've never heard of someone assigned to be a rear gunner as punishment. Depending on the severity of offence punishments could include "jankers" (confinement to camp, menial duties), being sent to the Aircrew Refresher School (the "bad boys course"), detention, demotion, or being drummed out of the RAF (either to the Army or to civilian life, prohibited from well paid jobs). The latter often resulted if aircrew refused to fly on operations. If a Medical Officer determined there was a genuine physical or psychological issue then there could be leniency, rest and treatment; if not the airman would likely be classified as having Lack of Moral Fibre (LMF) and treated rather harshly, swiftly removed from the squadron. There was a fear that LMF was contagious - that one individual "could start a rot which might spread not only through his own crew but through the whole squadron".

If the reason for going AWOL wasn't refusal to fly, after appropriate punishment crew position would be based on training; each role (on the standard heavy bombers: pilot, flight engineer, wireless operator, navigator, bomb aimer and air gunner) had specialist training, some cross-training was desirable in case of incapacitation during a flight but I'm not aware of anyone reclassified as a punishment. Rear gunners may have been slightly more vulnerable (I haven't got a breakdown of Bomber Command casualties by trade), but by and large bomber crew lived or died together.