Well, the latest synopsis says they have to tame Discord, not necessarily turn him good. The mirrors between him and Q just continue to grow.
First, I'm gonna repeat what Fhaolan said above: I can also see that if Discord is kept in check in mind rather than in body, then he won't cause any more trouble like he did in the past. But what intrigues me is not only why Celestia herself wants him put in check, but why now of all times. The synopsis says she's only just decided he needs rehab, so she may have something more planned for him than a simple "Discord may break loose again and we may not be able to stop him then, so we need to nip him in the bud." But I'm not completely counting on that.
Could this give credibility to the rumor that Discord is to become Twilight's mentor for the next phase of her studies? In his rumored leaked lines, he seems to be acting in a position higher than Twilight, yet judging from the next episode's synopsis, the Mane Six are the ones above him. This could either mean he's just being a troll as usual, or the lines are for a future episode where his and Twilight's roles from "Keep Calm and Flutter On" are switched.
Furthermore, how will the Mane Six go about reforming him, or at least interacting with him? If we take the rumored lines at face value and apply them to the next episode, he will be turned into a pony, possibly stripped of his powers. But should the lines turn out to not be in the episode, what then? Will he be let out of his prison, or will the Mane Six actually be let in?
Or maybe...what if it's all a lie? What if it's really Discord trying to trick the Mane Six into thinking Celestia wants him to reform just so he can escape, tear the Mane Six apart using Fluttershy as a catalyst, and plunge Equestria into eternal chaos? And even if he's stopped, he would still have bragging rights to say he finally broke Fluttershy.