Tito has been the idea man during the Pep era. He knows the players, which is a huge advantage with dealing with the huge number of games these players have to play. These factors will minimize the transition.
The club did not promote Tito to ignore him. All player moves will go through him, like it was with Pep. He won't always get his way, but his will be the main voice. Even if you don't like Rosell, you have to admit he is one smart cookie. He won't try to dominate his new coach, because that can't work.
Tito will have to show results. If the team drops off much, he'll get two years tops.
The big decision for Barca that led to all these cups the last few years and will lead to at least one big cup a year going forwards was not hiring Pep or hiring Tito. It was making it Messi's team. No big center forward is coming in, Messi will be playing there for a long time. Xavi, Iniesta, and Alves fit perfectly with Leo. Cesc was incredible with Leo for about half this year, that will have to stretch out next year. Villa and Pedro were fantastic with Leo the year before this. The forwards this year did not mesh as well, Cuenca was probably the best Messi complement, Alexis shows promise, Tello has some perfect skills for this job. I think a high scoring forward will be signed this summer, but it will be someone who can play on the side.