[Steve Mullins]"Not sure whether you can do this in AE"
[Steve Mullins]"automated app or something" [Steve Mullins]"but perhaps theres a new bit of software out that does this in a quicker ios-y type way"
Basically you would mask out the faces that you want to replace over the original footage. Then you would track the faces from the original footage and use that data to stick the new faces on them. A basic version of the Mocha tracker comes with AE and that would be a pretty good tool for this kind of thing.