little dudes to bigger dudes

So how to take a well-trained crew and transfer them to a higher tier tank, instead of break in in a new set of cherries?

So like there is a gunner for med tank tier 4 that is 108 skill and newb tier 5 gunner that is 89 or whatever and I'm getting tier 6 ... so do I get newb crew and train em up, re-train an older crack crew like tier 4 at 155 or ?

I would retrain the old crew. You can only move crews between same type tank. Arty for arty...heavy for heavy and so on