This is a great question!
You have to think of the MC as having a perspective or worldview that relates directly to their personal issues. In order to do that, you have to get a sense of what their personal issues actually are -- the thing(s) they are dealing with that no one else in the story is.
A Changed character will change that perspective -- actually change it 180 degrees. Whereas the Steadfast character will actually grow into that perspective, like their worldview has been tested and tempered by the events of the story, and come out stronger.
The recent Shrek thread is a good one to look it explaining the difference (there's a link to the users group analysis video, too).
See if you can explain Marty's or Po's personal issues in a sentence or two, and describe their worldview based on that. You may find that helps you see their "change" as Dramatica sees it, more like Growth within their steadfastness.
(One other tip. You probably know this already, but always compare the character between the beginning and end of the story; during the story they may waver or even completely change perspective and then change back, but it's how they turn out compared to how they started that matters.)