Need to do a last minute substitution?
Select the footage you want to replace in the timeline.
Then select the file you'd rather use in the project window.
Hold the Option (Alt) key and drag the preferred file onto the selected file in the timeline. Alternatively select the files and hit Cmd+Option+/ (Ctrl+Alt+/).
The files will swap out but any effects, masks, and keyframes will be applied to the preferred layer.
This is an excellent way to create multiple versions of a composition. To add footage to a composition, select and drag a file from the project window, or select the file in the project window and hit Command+/.