There are also many examples of pilots getting almost no training or information about their planes at the time of a switch. I've heard such stories in person. Things like (paraphrasing): "One day, P-51's were delivered and our P-38's were taken away. We were to fly the P-51's on a combat mission the next day. We told our commander, 'Hey, we don't have any flying time at all in these!' He said, 'Here's the pilot's manual. Read it tonight. You'll get some flying time in them tomorrow on our combat mission.'"
I heard one such story from a pilot transitioned into a P-47 from a P-38 in the Pacific and did end up getting into a fight that next day in his P-47 (as well as having a live duck in his cockpit at the time -- it's an interesting story), from a pilot transitioned into P-51's from P-38's in Europe, and from a pilot of P-51's who was put into a B-25 to go run errands (like picking up, as he says, "I'll call them 'show girls'" in Italy). That last one, folks asked him, "Wasn't it strange to hop into a B-25 to go fly it around when you were used to P-51's?" He said, "Well, I had a crew chief with me to help start it. After that, it's got a throttle, and elevators, etc., so it was no big deal. Do you have lots of training before you get into another model of car to drive it around? It's got a steering wheel and brakes and away you go. You don't think anything about it. It was like that." During WWII there was a much different scale to how people perceived risk.