I think account migration will have to come in two waves. It would be far easier to migrate accounts that are federated. That should be as simple as name swapping comments and posts and assigning them to the opposite servers while handing over user data.

The hard part is with servers who do not federate. Two solutions exist. One would have a data hop. It’s very likely that defederated servers would contain some link somewhere between federated servers. So the original server would pass data through one or two other servers to get to your new destination server that otherwise wouldn’t receive that data.

The other solution involves allowing a user to download their content with a key from the server and then present that to the new server. That data may need to be encrypted or have verification somehow to prevent tampering before reuploading it.

Any of these solutions will probably require manually approving and sorting through of data. The good news is that you can make requirements to say that users should have so many upvotes or whatever to receive a transfer to your platform. So their backup of data can sit still until you’re deemed worthy of transfer.

I’d say that data backups on your phone should definitely be an option regardless.