Mastodon Content Mover
mastodoncontentmover.github.io
mastodoncontentmover.github.io
A good way to look at it is how it is similar to email and moving between webmail providers.
But therein is the distinction between decentralized and federated. In a perfect world, my identity is owned entirely by me, independent of any central server. If I choose to have a different server host my data, all there should be needed is for both servers to authenticate my identity and confirm my transfer authorization and sync would be seamless. A read-only replica server should also be possible (selfhosted or random servers). I don't knoe the technical reasons why they don't do this.
It also can't set the date correctly.
All in all, maybe it would be better to archive your old posts somewhere other than your new Mastodon account and instead put a link to it in your profile? Do archive sites exist? Maybe someone could create a tool for that.
If you just want to archive them for your own purposes, you already have the archive download.
The point of this is fixing the biggest problem of account migration: loss of account history.
It's likely that nobody will see your old posts anyway because they won't scroll past the first page of your profile, and you'll soon be adding more posts on top of them, so they'll just get further buried. You'll need a pinned post for anything you want people to read. Clicking on a link is easier than scrolling a long way into a feed.
This is largely a matter of motivation: have you explained why they should care enough to click? A link that just says "archive" probably wouldn't be motivating.
Meanwhile, if you move the posts, all links break, and those might be links from places that are more motivating. It might be unavoidable the first time, but it would be nice if it didn't happen repeatedly. I expect a Github repo will still be there after today's Mastodon servers disappear, for example, so it might be a good way to archive?
All of this is assuming that you have archives that are interesting and valuable history and you want them to be public, so your archives are worth linking to. It's much like archiving old public mailing lists. Maybe similar archiving tools could handle both?
But if the posts aren't valuable or you don't want random strangers to be able to read them, it might be better to just let them go, or not put them online at all.
Was kind of surprised to not see any reference to this app doing "All the heavy lifting" or something like that. https://www.youtube.com/watch?v=j9QwaWXsbkU Come to think of it, I'm surprised their songs aren't used more for wordplay related to the fediverse.