Posts are federated already, so you can’t migrate them. The content from your old account is already on the new server, just under your old account name. Importing posts to a new account, especially if they “posted” from that new account isnt possible.

It would be great to have a “migrated/archived” tab on an account that would bring over posts/content in a local only way that was just attached to that account, but that’s not a feature lemmy has.