We plan to work on a possible migration script or so at the end of the year. But it will take time and I am really not an expert on these things. If you want to help out investigating the database structure etc. you are more than welcome.
I am currently thinking that a stand alone python script to copy over the data from a Lemmy database to a fresh Piefed instance would be the best, but migrating the pictures from Pictrs would be another problem then...