How to set a new GPG key?

I'm going to use a new GPG key, letting my old key expire. How can either add a second GPG key to keybase (which I'd prefer) or set a new GPG key? (So far, I have only found UI indicating that I can update an existing key as long as its fingerprint has not been changed, but not replace it.)

Start with the existing key-management UI described in the issue and trace how it handles updating a key whose fingerprint is unchanged. Confirm whether the intended outcome is support for a second GPG key, replacement of an expiring key, or both; done means the chosen workflow is supported and documented in the UI.

Written by the indexing model from the issue text.