Anyone who called Mastodon instance migrations "seamless" has not actually ever migrated. They are "possible" at best.
First, you need to set up the new account's preferences, filters, description, links, and pictures. Manually.
Then, you export six CSV files and import them one by one.
Now your friends see the follow notification and follow you again (?) or text you in confusion.
Then you make the old account a redirect, the new one an alias, and then you trigger the followers move.