I made it so a hidden type variable is included in edit_in_form in new records.
http://test.dwagn.org/wagn/action_items+*tform includes "+due date|type:Date" and things look fine when adding a new card http://test.dwagn.org/new/action%20items but after it's been created, the +due date card is Basic rather than Date, and it has an underscore in its name rather than a space.
On cards created before i added "+due date" to the form, when i click on "Add +due date" it does get created as a Date card, but still has the underscore.