How would I do this in After Effects?

It’d be easiest if you used a green screen. That way, you could treat the background and subject as two separate entities.

As already said, use a green screen. Make one video with the guy in front of a greenscreen and another one with just the background you want him on.
Desaturate the background video, put the greenscreen video on it and delete the green part of it.

thanx, but it’s too late for that, the footage has already been shot, I learned that there is a way in AE to make a mask that precisely cuts all around the talent, but then you have to toggle frame-by-frame and adjust as needed. We ended up just cutting a vignette mask around ‘em and softened the edges and they were happy with that.

I appreciate your input though. Thanx.

Oh that’s a good idea. Well done!