There have been postings (brk postings for the most part) on the subject that Buffet will tell DQ to start serving Coke instead of Pepsi.
I partially disagree becasue this has come up before---I think it was that Sees Candy would not accept American Express cards. When pressed, Buffet said that he buys business that have great managers and then lets them run the business. So it was not his job to tell or order them what to do. It was against his philosophy. So I find it highly unlikely that you will see that Buffet ordered DQ to serve Coca-Cola products.
On the other hand, great managers are great managers because they they understand the big picture. And the big picture here is that ultimately your boss happens to own a big chunk of KO and he likes it. I suspect DQ top management will smell the wind without being told. Of course, then they have to convince the individual owners of DQ franchises that KO is the way to go.