There's no "set" schedule really that all shows, all networks follow. Let alone when they announce.
I'm sure TNT is watching Dallas very closely. From what I've heard, critically it's doing well, but the numbers have been down since coming back in the winter instead of the summer (when, arguably, "nothing else is on") ... Hagman's passing is obviously foremost on the network's mind as an effect of how much longevity is left in the show.
In short, we'll find out when we find out.