The difficulty of this question is to quantify a large change program that you have managed with enough detail
I think the key to this question is split up your answer in terms of what is the change that will affect your relationship with customers and then go to the how you did it part. The how part mainly revolves around research and good communication skills.
Split into whether I had identified the problem, or someone had come to me. Identify, plan, process map, receive approval, update if necessary, implement. And to ensure all relevant teams were kept in the loop.