Is there anything connecting the posts to the users? Is there some type of a relationship field that is populated?
Is there some unique information that exists for both the post and the user?
Without some way to know what post goes with what user the only way to do this would be manually.
But it there is something that connects them you could create a temporary function that will do a query to get all of one (posts or users) then loop over them and then find an update the other end for each.
