Changing the Master Page of an exisitng page

  • Hi,

    Does anyone know it if possible to change the Master Page once a page is already setup.

    When I create a new page you can select the Mater Page from a dropdown of the ones available, but I can't see an option to edit it anywhere.

    I ask as I need to be able to swap between two versions of the homepage, one with an alert popup on it, and one without.