I think I know the answer but just to be sure, I ask here:
It's clear what action is required to change to the animal form (or elemental or etc). But which action is required to change back? The same one? Or the same one and every more 'expensive' action? Or does it defaultback to the standard action?
Based on the wording of the feat, I'd say you can ALWAYS (regardless of how you initially shifted) change back to the Druid's original form as a swift action with no penalty. The feat allows the swift change and doesn't list any penalties for form -> regular.