Hmm. That point about Andy Dufresne makes an interesting point. One thing you can do to make the change seem less obvious is push a Steadfast character to the brink (or conversely, drive the Change character as far from changing as possible). We may have an inkling that the story is about the Steadfast character remaining Steadfast, but in that moment when everything has broken around them, there may be a hint of doubt within us. Similarly, a Change character can climactically burn all their bridges and refuse any progress, and for a second, we might actually believe them. This is a little cheesy, true, but it can be done well.
Another option is to do the "back-and-forth." For example, a Steadfast character might go most of the way to change early in the story, then climactically realize the error of their ways and go back to the way they were. A Change character might change at the start, temporarily go back to the way they were before, then climactically change for good. A Steadfast character might change, then revert, then change again, then finally go back to normal. Or... you get the picture. Do this right, and at the moment of truth, it won't be obvious which way the character is going to fall. How convincing was that change before? At the moment of truth, are they willing to stick with the path they'd chosen before? As they say, it ain't over 'til the Limit runs out.
I think, by the end of Act I-ish, it'll be clear to the audience what your message is. The question the audience will struggle with is: will the characters follow through with your message and reach a Good ending, or will it all end in Tragedy?