Multiple controllers with one route

I have a controller which I want to deprecate but the deprecation can't be sudden. I will create a new controller and slowly migrate users from old controller to new controller. I don't want to change the routes, basically same route should be able to direct to old controller or new controller depending on a flag. Currently I am thinking of using redirect_to to give the control to the new controller from the old one. I want to figure out the cleanest possible way to do this. Any suggestions are welcome.