It is not possible to change roles in the middle of the "shooting" as the story unfolds, there is a grand plan(you mentioned that somewhere), there is a loyalty test transpiring, a check and balance, God made it so, such that, the "Pride" in that "being" satan- is of such a creation that is difficult and hardened - just as God can harden the hearts-minds of the Pharoah. In layterms, the "software" and "program" that was installed-embedded cannot be retrieved for now to effect such changes. It was designed such that, even if you changed the main-frame software-reprogam, the hardware - it has already mutated to the roots/genes/dna of people-recruits as well as evil continue to propagates like a disease-virus. If there is a chance to change would only take effect on the "Supreme Directors" call. It would mean, a total overhaul, and get so many production staff involved - if it were a movie- it would call for change of the whole plot, story-line, script-writers, actors/actresses and stages, almost everything - it would be very very costly investment for heaven. Such that it would be better to wait what happens the next thousand years when spiritual government takes place.
Another way to look at it is like watching the football game, Can you imagine that in the middle of the game - you ask for the opposing "winning-side-kuno" to stop the game - and change the rules, what do you think all the audience and players would react - both "seen and unseen". This is more than just "reality TV or tru-TV" that unfolds - from the otherside of the spectrum.
Khen, unfortunately, I know you were hoping to come up with a topic-discussion to show the struggle between good and evil and stimulate discussion(internal-external). I know that for many this school of thought or line of "thinking-not really mine" might be a surprise because my discussion is not the usual pattern that follows in the forum. Evil was already hard-wired(firm-ware) in Satan. As you can see without protagonist/antagonist role - there would be no movie, entertainments, no cliff-hangers, thrills - all songs and dances would be boring - heaven needs a challening entertainment. Just as we enjoy our local entertainment....
What comes out of this heaven-made entertainment is how we well we played our roles - best actors, actresses, supporting actors, best film, best musical, etc., come judgement day...believe it or not..