I haven't had to change Ts yet because I was moving, but I have have to change Ts abruptly because she was moving. She only told me 2 sessions in advance. That was incredibly difficult. I ended up writing her a thank-you card, and using the last session to tell her how much she had helped me. She referred me to a new T, who is the T I am seeing now (It's been 3 years!). In retrospect, it was a blessing in disguise because (even though I was resistant at first) my new T has turned out to be even better.

Leaving my current T is unthinkable at this moment, but I know it will happen someday. Probably within the next couple of years. At the moment, I am happy where I am at but I know I will not be in this city forever. If a compelling career opportunity opens up elsewhere, I will move. In my field (academia), there is often little notice between when you find out you've been offered a position, and the position begins. So, in that event, you have to pick up your life-- say goodbye to T (and everyone else)-- and move fairly quickly. If that happens for me, it will be a mixed blessing. Like you, leaving T would be one of the biggest downsides of moving for me!