Best practise for change management

The purpose of DevOps, is exactly what you need. But it requires, your team to properly work together with the client, and that you keep listening from your clients in time. The thing about big companies is that, they have several clients and they keep listening continuously, and that keeps them focused on their work, unlike hearing back too often, which (as in your case), causes a lot of trouble as team itself looses focus on the problems, features and bugs in the software. 

Visual Studio Team Services would be a viable option here for you, and also, it would store all of your backlogs, iterations and client-team communications easily.