tom23 I don't know if there's something that automates this process. When I needed to do something similar before, I used this extension:
https://discuss.flarum.org/d/21731-discussion-and-post-author-and-date-change
With this extension, I moved posts from one account to another. I wouldn't say it does exactly what you're referring to, but it might be useful if the number of posts to be moved is small.
