I have not checked how moving the active personal account from one Fediverse server to the next actually works, technically. How does the new server get my follower list?
It works totally separately from the CSV files, because if it relied on CSV files people would abuse it blatantly. Spammers would upload CSV files of thousands of people who never actually followed them etc.
So, to prevent this abuse, transferring followers is done by a totally different process.
@FediTips@dj3ei you can do it within the same server too, it's somewhat common for people on our server to change callsign and also change username using this process.
upgraded license. Many countries have basic, intermediate, and full license which require increasing skills/tests. It's common for someone to upgrade and get a new callsign for the new license class.
moving. Your callsign is specific to the country issuing it, so if you move to a different country and get a new license you might also want to change username.