I think my new T has the file from my old T (same office so didn't require any action on my part). As for explaining what you have talked about in the past, I find that I feel a need to do so because of the size of the file and that I don't know how extensive the first T's notes were. So, I prefer to explain some of the background. Besides, my T's that I have had don't always seem to remember things as well as I would like. I suspect she doesn't have time to review the file a little before each client's session. So, a few bits of review to make a story or emotion that you are sharing is something that I prefer to do.
Since you do more work that the T, I doubt that you would have to start over. I hope you give the new T some time to see how he/she works with you and whether or not you click. If not, look around.