So I wrote a little web app that allows a user to move their user data, like settings and subscribed/banned communities, from one account/instance to another.
It runs completely client-side, but is hosted on GitHub for the moment. Maybe it'll be of some use!
Features:
Export user data from any Lemmy instance (>=v0.19)
Download user data as a text file
Modify user data in the browser, e.g. to add or remove followed instances
Transfer user data to the target account on the target instance