I have four of my kids playing softball and I know how difficult the changeup can be. I'm sure you've heard the old adage, "think fast, adjust to slow". While it is true that a quick mental adjustment is needed to hit the changup, there are a couple of things you can do to make it physically easier. First, your muscles have to be accustomed to hitting a changeup. This can only happen through practice. You can buy equipment that helps you practice, such as 'the hands back hitter', or you can accomplish the same thing by having someone softly toss you the ball, making sure that they don't flip it until you've taken your stride.
The next thing you need to work on is your stance and stride. A long stride and too much of a weight shift to your front leg will lock your hips and make it harder to adjust. Work on not locking your knees, balancing on the balls of your feet and taking a stride of no more than 2-4 inches. There are detailed instructions on this website: http://www.syasoftball.com/hittingfund.html - the important points to look at are the stance, the negative movement (leaning back toward the catcher), and the stride.