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.