Unchecked it quite right. In the past where there have been rapid transfers it has probably been in time of pilot shortages. In the 90s there was a successful nav-pilot crossover but once there was a sufficiency of pilots the scheme was dropped - why train someone twice?
One commercial operator, now defunct, did not even retrain his existing aircrew when he got a new type in. It made sense to let people highly experienced on an old type retire and recruit new blood trained on the new type.