I had to find a new T when my old T became my colleague at my old job. And I found out about this (had no idea it was coming) at a big meeting, he didn't even tell me himself.

I also restarted therapy last year after a 15 year hiatus. With both new T's, it was really not starting over again. You just kind of pick up where you are, and if for some reason either new T feels or you feel the need for some background, you can provide it. But it's not a do-over, in my experience, but I think that conceptualizing it that way makes getting a new T seem more daunting.