Currently you can download the list of people you follow, but have no way to migrate the people who follow you. If you're simply moving then you can leave a forwarding address, but that's currently the best you can do.
People are actively working on it to ensure that people can move across instances smoothly, but it's not yet there.
Person Person X on instance I (X@I) follows Person Y on instance J (Y@J), and Person Y moves to instance K (Y@J->Y@K), there is currently no automatic way to have X@I start following Y@K.
Y@J can announce they're moving and say where they're going, but there's no automated process. Several have been proposed, all (so far) can be horrendously abused. So far, X@I has to see the notification and follow Y@K manually.
People are actively working on it to ensure that people can move across instances smoothly, but it's not yet there.