Have you tried a transformation like this before? By the way, if the sky replacement doesn't work for you, you can manually select and mask in a new sky - this takes some practice, but do look up 'perfect selection in photoshop' and you should find tons of videos showing you how that is done. The upside of such manual selection is that you are able to keep small plants or branches that are within your transition area. In a dark image like this, I don't mind, but a different or brighter sky might need it. Either way, have fun!!