Services, maps, and apps have the URLs they are created with written to disk. While redirecting may work for the home page of Portal, none of the content stored within Enterprise will be useable. It would also break your Hosting Server federation, because the URL is saved within many of the setting / config files. To change the public facing URL, you would need to standup a new Enterprise system, then migrate the content over (with Esri's help or on your own).