That's my only fear with this episode. No matter what, I want Discord to be the carefree spirit he always has been. At most, he won't be uprooting houses and turning the clouds into cotton candy or turning ponies against each other. If he retains his personality and traits, I am fine with this episode.
However, there needs to be SOME purpose to his return, because Celestia being a "forgiving soul who believes in second chances" won't work at all. Discord had been imprisoned twice, so it would be something important for her to do something as drastic as return a powerful deity such as Discord.
That's exactly how I feel! They can't bring Discord back JUST for the sake of having an awesome villain return. (I never considered Discord evil, but I don't want him to suddenly become all "good", either.)
I'm also irked because my friends and I have been working on a Friendship is Magic comic for a while and it mainly focuses on Discord coming back, and we are all worried that this episode will mess up our plot (we have considered loop holes to work around possible things). (This is also a Discord x Pinkie comic ._.) Now, I'm not one of those people who's going to throw a fit just because the show does not fit my head-canon, but I just hope everything with Discord's return makes sense and is pulled off well.
Don't worry, the fandom has more than enough fuel to last for several more years without the show, and Hasbro will doubtlessly carry the lessons learned into Gen 5. It will be intriguing to see what they can do.