Currently, we have 3 primary web apps all with their own look and feel and backend systems. In some places, the apps try to solve similar, if not the same, problems. A number of our customers subscribe to more than one of these apps and have to deal with the differences in the systems and the duplication of effort and data. We're working on a multi-year project to merge these apps into a single solution. Better for the users, and better for us. As each phase is completed we're releasing the improvements to our user base.
As we get deeper into this project I'm becoming more worried about the end result looking like it belongs together rather than something that was developed over an extended period of time. How do you prevent each phase of the redesign/conversion from having its own look and feel?