If you are using the spark view engine, you could probably do it with a variation of the theme filter (http://sparkviewengine.com/documentation/viewlocations#Extendingfilepatternswithdescriptorfilters). For each new visitor to the site, determine if you want them to see the existing or new version of the site and set a cookie. Wire up a descriptor filter that looks for the presence of the cookie and modify the view location to look in the folder containing the modified views. If an alternative view exists, the Spark engine will automatically render it in place of the "normal" view, otherwise it will render the normal view.
If you are using the normal WFVE, then the simplest way to manage this would be to define a folder under Views where your view alternatives live. When you want to provide an alternative view, you place it in a location that matches its position within the normal Views folder but rooted at the alternatives folder e.g. to provide an alternative to Views/Users/login.aspx place your new view at Views/Alternative/Users/login.aspx.
With a convention in place for locating your alternative views, you can extend the WebFormViewEngine and overload CreatePartialView / CreateView to inspect some aspect of the ControllerContext to determine whether to render the default or overloaded view and alter the path as appropriate e.g. changing .../Views/Users/login.aspx to .../Views/Alternative/Users/login.aspx.
