I'm not sure how this really changes anything other than ESPN now being in a position to want more Big 12 games possibly. There aren't any more games to go around than there were, and if CBS and NBC are getting further in, that only opens up slots in the ESPN and Fox universes.
The only real downside I see is having a partner who's tried to harm you in the past, but that was done specifically for the sake of getting OUT into the SEC. I don't think ESPN has an irrational desire to destroy the Big 12, their previous assault was a means to that end. I think it's likely that there's the possibility of a mutually beneficial relationship between the parties, and I trust Yormark to be able to sort that out.