Hello - I am new to this site/forum, but I believe I can provide some info to your query.
My maternal grandfather (John Dennis) was an air gunner in 76 Sqdn out of Holme on Spalding Moor during 1943. He was normally 'tail end charlie' in DK195 - pilot was Alf Kirkham. This aircraft was shot up during a raid over Mönchengladbach 30/31 August 1943 but the pilot & navigator, altho' both injured, managed to return the plane to Bradwell Bay, Essex, where they crashed. (
Halifax V DK195 [Royal Air Force Aircraft Serial and Image Database]). DFM's were issued to the pilot and navigator - who were hospitalised and out of action for several weeks. My grandfather, unhurt, subsequently found himself without a crew - so was 'kicking his heels'.
He subsequently got the call to stand in for a mid-upper gunner (who had gone 'sick') on a mission in EB253 - pilot Birger Fjaervoll (Norwegian), over Hanover, Germany. Aircraft was shot up and was on fire and heading downward. One crew member managed to bale out, and all except my grandfather, were killed. During the crash he managed to get out of the turret, went aft to see if he could help the rear gunner get out, but the aircraft lurched downwards, throwing grandfather forward against a stanchion, smashing his leg/knee and breaking his back - multiple breaks/fractures. He somehow survived the crash, woke up in the wreckage looking down the barrel of a German gun and immediately asked for water.
The Germans, to their credit, hospitalised him and basically rebuilt his leg before sending him to POW camp - ending up in Heyderkrug Stalag Luft VI before repatriation in 1944. He had to wear a built up shoe to compensate for his shorter leg for the rest of his life. So, in answer to your question - it would appear air gunners were cross trained to operate in both upper and rear gunnery positions. If he had been in the rear turret that night he would not have been able to tell the tale. Hope this helps.
Best regards
David Gilchrist