as far as I know from Air Gunner of 311 Sq:
1) training was the same for all of them, position was determine at OTU but they must be able to serve as any gunner in the crew
2) when they were flying Wellingtons, they were more or less still in the same positions, but several times they were exchaged by CO (for example most experieced AG was flying ar rear gunner with freshman crew), sometimes they change positions spontaneously (official record states that A was front AG, B was rear AG but the plane was shot down and rear AG perished but it was person A not B).
When flying Liberators (not BC but CC), in most cases rear AG was in his turret but WOP/AG, WOM/AG and beam AGs were changing places approx. after 2 hours of flight.
Hope this helps a little