When Making a Comprehensive Retrofit of your Code...

chizor asks: "My programming team is considering making some sweeping changes to our code base (150+ perl CGIs, over a meg of code) in the interest of consistency and reducing redundancy. We're going to have to make some hard decisions about code style. What suggestions might readers have about tackling a large-scale retrofit?" Once the decision has been made for a sweeping rewrite of a project, what can you do to make sure things go smoothly and you don't run into any development snags...especially as things progress in the development cycle?