Once "Alternate Input Slot Mode" is enabled on a Series Slot (not shown), switching between different modes could be implemented as a combo box in the Open Slot Dialog.

Alternatively, such a combo box could be put in the analogous place in the Slot Configuration Dialog. (See below, but not mocked up as described here).

In either case, operating the combo box would replace the contents of the dialog with the appropriate "Open Slot" or Slot Configuration widgets.