Transitioning to Lightning Experience doesn't have to be a big bang effort. Instead, simplify your rollout by moving users over in phases. The benefit is you can tackle business and technical requirements incrementally, and iterate on lessons learned.

Check out the new Lightning Experience at Your Pace video (http://salesforce.vidyard.com/watch/p9ajTfV7Ude1NJTfct1Maq)

to see how you can use permission sets and profiles to tailor your rollout by groups, teams, or whatever else makes sense.