I think this should be an UI feature. Wordpress has always offered this: When deleting a user, I am being asked what to do with his/her posts (i.e. who these should be assigned to). An auto-complete dialog could just ask for the username, and then there could be a task in the background, assigning all deleted user’s activity and content to the new user.
Scenario: Old legacy forums with lots of baggage sometimes have old user accounts with massive amounts of content that should be carried over to newer user accounts. It can’t be done by hand and for those of us on hosted Discourse installations there is no access to the command line.