When I looked at the crew details for AJ-C - skipper P/O W. Ottley - it said that it wasn't known why the two gunners - Sgt H.J. Strange(Front Gunner) and Sgt F. Tees(Rear Gunner) - had changed places. I can tell you why.
Firstly, on normal sorties, the Bomb Aimer manned the front gun turret and when doing so his feet would dangle down to where he'd be lying in the bombing run. On the Dams Raid, the Front Gunner's feet would be right in front of the Bomb Aimer's face. Stirrups were put in place to keep the Front Gunner's feet out of the Bomb Aimer's face.
Sgt Tees was a big man, bigger than Sgt Strange. Thus they agreed to swap places. We all know the result!
I read in Helmuth Euler's "The Dams Raid - Through The Lens" that in March 1982, Sgt Tees committed suicide after being racked with guilt that he had lived and Sgt Strange hadn't because they had switched places. This was also told on the 60th Anniversary trips. I did the one in October and Bill McQuade, our guide, told us it when we went to the graves of P/O Ottley and his crew. Sgt Tees asked that his ashes be scattered on the graves of his crew. At least they are all together now. I also saw a model of the fuselage of a Lancaster and saw how it would have been for the Front Gunner and Bomb Aimer with the Front Gunner's feet dangling in front of the Bomb Aimer.