The show can't take two years of being the Dorne and Cersei show. While you are changing short term plot, it stays true to the characters and I can't see the side stories changing the major arcs.

Jon showing up and basically leading the Nightwatch in one felled swoop in the books is cool, but he always suffered being so far away from the action with his hands being too clean all things considered. Travelling to Casters and the pending assassination attempt is a nice sidetrack.

Biggest change I see happening is he runs into Bran. In Storm of Swords Martin fucked with you on how they JUST missed each other during that storm in the tower.

However, considering Martin pulled the "The Starks children ALMOST got reunited then fate fucked em!" more then a few times in the books I don't mind the change. I think Bran meeting with Jon even if its just a quick hug, would serve well to ground two characters that are in their own universes away from the action and gives the tv crowd a little bone considering it took like one episode 4 years ago before the split all the kids up.