As of 2015 you can do this yourself, and you don’t have to email anyone.
Go to your plugin page (example: https://wordpress.org/plugins/hello-dolly/ ) and click on the ADMIN button on the right hand side.
On that page, add the new person as a committer and remove yourself.
Then update your readme and plugin files to point to the new USER ID and check the code in. You do not need to bump your version numbers.
Done 🙂