To move crew from one vehicle to another, you must retrain the crew for the tank you want to move it to. You may either use Silver to have your crew start at 75% training, or pay Gold to have your crew start at 100% training.
Transferring to Premium Tanks
The transfer of a crew to a Premium tank, or from a Premium tank back to the crew's previous tank is not subject to any fee.
However, the transfer of a crew from a Premium tank to any other tank is subject to the normal fees. If the Crew XP is not at 100%, you will be offered to upgrade the Crew Training for Silver or Gold.