Designing A Renovation Project

We always think about new construction, but what about a renovation?  What are the challenges of trying to make a client’s lighting better when you can’t start from scratch?  Can changing lamps (bulbs), adding lenses, aiming and similar techniques be enough?  How do you analyze what is needed (not as easy as just saying “more light”)?  How do you explain and defend your design suggestions?  And how do you look ahead to what the challenges to implementing those design improvements would be?