I'm worried about what you're worried about - even before it has happened.
I want to move out of state so badly, but then I risk losing my T. In the state where I currently live, my T isn't allowed to continue with telehealth or video appointments at all. The rules are per state, and so this state sucks all around.
So, I stay. I stay to save money, pay off credit cards, and process more things with my T.
I'm honest with my T about my feelings, even if I'm clingy with her. She's okay with that. She knows how to handle that, and we work those things through. I think my being open with my T has brought us closer, and has allowed me to feel more secure and less clingy with her - over the period of 2 years that I've seen her.
I think if you are honest with your T, it will help when it comes time for closure. Hopefully you can bring all these things up so that your T could help you transition more, and spend more time with you before that transition transpires. Maybe your T could offer a monthly or quarterly check-in.

Asking your T what it would look like to transition and then to keep in touch after the transition would help to put your mind at ease. That isn't clingy; that's more planning and communication. I think your T would understand.
It's also okay to be clingy with your T. Your T will be able to help you with that, and with the transition, too.