Why not just copy the configs over?
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That is what I planned, but after reading around a little it does not seem to be working that straightforward. Plus I wanted to use the opportunity to change my folder structures of my libraries a bit and this seems to also mess with things like taking over metadata for files. Same goes for user data it seems.
Found an old answer from 4 years ago where a Jellyfin dev said that migrating accounts and whatever else is on the roadmap but way down the line.
I mean, that's how I upgrade my dockers, the configs are mounted outside the containers and I spin up an update docker and it picks up the configs no problem. Maybe just try it out.
The item IDs are tied to the actual file paths, so don't change your library structure at first. Otherwise you might still have the user data, but it can't be matched to your media.
Generally I've not found a way to transfer things. If you find one, please let me know...
Not tried it myself, but you create a trakt account for the user, sync it then use that to preserve watch history on the new instance.
If thats the easiest way to do it, that would be a bit sad.