> Please note that Mastodon can move your followers, but it can’t transfer your old posts or uploaded media to your new account. If you want to keep a copy, download an archive.

Sadly that's still an open issue that from time to time gets new input, but doesn't have a lot of traction. I feel like that's a big blind spot of Mastodon to this day and also basically forces people to stick to the largest instances that are less likely to go away, defeating the purpose of a distributed social network.

https://github.com/mastodon/mastodon/issues/12423

There's this if you want an automated way to download and repost your content: https://mastodoncontentmover.github.io/

It's also a source of power for mods.