Just for the record Jon of waindigo made me a little mod to change the original Join date for a user. Only took him a few mins so this kind of thing is quite possible. It was for users I'm importing from an email list.
Another route to do what the OP asks would be an addon for admins to Log in As User. I had this on another forum software and it's very useful. It needs to allow the admin to log in ADDITIONALLY so they are not logged out of the admincp which gets annoyiong having to re-login twice every time.
With such an addon I could use the frontend to switch IDs then post or otherwise act AS IF I were the other user. Excellent for sorting out newcomer messes.
In the OP situation I'd copy the post in as the desired user, Delete the first one.
That just leaves the later date? I don't know if that is important here.