[BomberBoy]

My campaign follows (or at least it will if I continue with it) an American B17 pilot who completes his tour in the UK and goes for re-assignment to B29's in the pacific.

That's actually very plausible. Col Paul Tibbets flew in some of the first B-17 raids in the ETO with the 97th BG before transferring to B-29 testing. After that, he helped form the 509th Composite Group and flew a very well-known plane called the "Enola Gay".

If you're going for more of a "what-if" scenario, there were a number of individual aircrew that went through the procedure you put forward. There was even one complete B-24 group (the 489th BG) that was withdrawn from the 8th AF in Nov '44 for re-training, re-equipping and re-deployment to the Pacific. (They did not make it there before the end of hostilities.)