What happens if a teacher moves?

Answered

Many students gain recommendation letters from faculty who switch schools or retire by the time the student applies to college. The status of the teacher is less important than what the teacher can say about you. They might start off their letter with "Tw years ago, back when I taught at X school, I taught Y student about Z."

A good place to start would be to shoot the teacher an email once you're ready. Get their email before they move so that you have it just in case you don't know how to otherwise get in touch.

Ask your college counselor if there are any special instructions for that teacher to upload a recommendation to the Common App or whatever portal you're using to transmit recommendation letters.