Then when both of you are using HIS plane: He is holding HIS transmitter with the correct crystal already installed for the receiver HE installed in the plane. YOU are buddied in and holding YOUR transmitter(or buddy box) which should be turned off anyway and does not even need to have a crystal installed. Whenever HE holds the trainer switch, your sticks control his transmitter using HIS crystal. I think it is fine, but unusual, that he wants you to crash his planes for him. Most instructors prefer to teach on the students plane, that is the one you need to learn how to fly. It WILL be different from his, be prepared for that when you transition over. Do you understand all of this? I do not understand any reason why you want to move crystals in this scenario