I've updated the guide on how to transfer your Mastodon account from one server to another:
➡️ https://fedi.tips/transferring-your-mastodon-account-to-another-server
It now has an additional step where you switch off email notifications for new followers on the account you're moving to.
Apparently some people have been receiving lots of email notifications for new followers when they've transferred followers over. For some reason no one has raised this issue before, but it seems to have happened to a number of people.