In this Shot here (Below):
I want to replace the entire Sky of this Scene with a picture of this Sky (Below):
So is there a way to do it The Fast Way, and not The Long Way?
Whenever I need to create a matte I tend to look for the easiest solution first. In most cases involving the sky you can create what is called a procedural matte using the color channels, color controls and matte tools. It's called a procedural matte because we use a procedure instead of doing it by hand. In many cases the camera is also moving in the shot so some tracking may be required.
This tutorial shows how to use Set Channels and Colorama to create a luma matte and Mocha to track camera movement.
My technique for creating a matte from the sky is a little different. Take a close look at this screenshot. It demonstrates the creation of a procedural matte from your shot in under 5 minutes. The black solid is just a garbage matte for the foreground. Pre-compose this and use it as an alpha matte for your shot and you should be good to go.
This may also give you some ideas on background replacement and tracking.
