I have only once done this and it was for a pdoc. I did this because a lot was going on at the time and so I felt it was easier for him to get the information that way quickly. But I have never done that with another T or pdoc. You can decide how much information goes to the new T. Do you want a summary of what you have been working on and your diagnosis? Or do you want the new T to have the session notes from every session you had? So it is up to you how much information is transferred. Whatever you do is up to you.