I'd like to implement the UA rules for Warforged which allow a player to change their natural armor type after a long rest.  Essentially I'm hoping for a way to just add the dropdown in the features list of the character sheet and have the selected option automatically adjust the AC calculations of the character.  The only place I see options for are in the character creation screen.  Is there a way to implement these into the sheet instead?

No, for features like that people just have to go to the character builder to make their selections.