Handling post-registration action
It's not a good practice to put everything into the request parser. You can put there your formParser, but let an UserController.* handle the rest.

Think about your userActions (UserController.updateUsername) in a more abstract way, for example if you decide to setup an admin backend, where you have some other routes, you would like to reuse your code..

If you consider, to use a seed script, it could directly talk to the UserController, or modify values from a CommandLineTool.

The best scenario would be, if your routes just points to the controllers, so you could define a bunch in a single file, to see all your endpoints, and where they are pointing too.

One more hint: use a middleware to parse the form data.