I have a project within which I have a complicated composition with lots of layers using the same video.
I'd like to do exactly the same stuff I did in that comp in another comp with a different video, and without changing the first comp.
Soi I duplicate the first comp, and then..?
Import the replacement footage into the project.
In the timeline window, select all the layers that need to be replaced.
Alt-drag the new footage from the project window to the timeline window.