How do you execute the redesign of a web application with nearly 1,000 views? My gut response is to avoid changing markup and do as much as possible in the CSS. With a totally rebuilt, responsive pattern library and massive UI changes (like the subscriber table wizardry Mardav talks about below), we knew that wasn't an option. Indeed, every view had to be touched. In some cases that just meant swapping a few classes, but a lot of pages had to be re-built from the ground up.
No matter how much prep we did outside the codebase, we knew that process wasn't going to be completed in one of our standard 4-week release cycles. Our new views and stylesheets had to coexist with the existing views and styles through at least 2 major releases. To do that, our devs created a stored user-preference for "usingNewDesign." If that setting was true, the app would look in a "new-views" folder for the requested page before falling back to the standard views folder.