Overturning the Red will be an admission of fallibility by the FA &, by implication, the ineffectiveness of the Ref in question. In order to save face and sheer bloody mindedness, I think they'll leave the red in place...call me pessimistic (just don't call me late for dinner).
Now, if they were to dismiss the card, I would rather they took a long hard look at themselves, and establish some new ground rules for derbies. Clearly, video technology would have prevented that red and, arguably, it may have prevented the missiles being thrown at Suarez (?). They know the return leg will be volatile, they know tempers will flare, the Police know it, the Clubs know it, the Fans know it...but can the FA afford to stand by & wipe their hands of all responsibility? I think they could at least start to defuse the next derby by saying "Yeah, we think we ****ed up. Sorry, lets move on & get it right next time."