In reply to Chris Sansum:
I'd use a gradient in photoshop. Save the original file as a new file. Increase the canvas size vertically, create a new layer to put your new sky on, select the correct gradient tool (foreground to background). Pick the sky colour just below the top of your current image as the foreground, then within the tool option, select the background colour as something slightly darker. ( or whatever seems appropriate) Click on the original sky just below the top of what was the original upper band and draw the gradient up to the top of your new canvas.
It may take a few attempts to get it looking convincing but with a bit of trial and error it should be fairly straight forward to do. Good luck.