My DD used a "circle change"--which is simply turning the wrist over at release so the back of the hand points to home rather than the palm. But, she rarely threw it. What she did do was vary the speed of her pitchers, and threw a lot of off-speed stuff with her drop. Her drop would vary between 50 mph and 62 mph. She could throw the drop for a strike. Her fastballs would be between 62 and 65 mph.
I'm not sure a "pure" changeup is very effective for good college softball. Hitters are practicing 1 to 2 hours a day. So, they generally have quick hands and keep their weight back. If a hitter "gets it right" on a changeup, the ball is gone, and the game is lost.
As a X college pitching coach I did not enforce one change over another. If a pitcher came in to the universiy throwing a good change up I never changed it. But I have had some of my students who went to college had their change up changed to the change up the pitching coach threw when she pitched. I teach different types of change ups because not everyone can throw one type of change up.
My approach to development of a good change is to establish certain commonalities in any good change: Using the same delivery and motion as the pitcher's fastball and other speed-based pitches. Strive for a speed that isn't too slow (which allows the batter to reload) rather looks fast coming in, but isn't. Strive to locate the pitch -- always make it look hittable as it reaches to 10-15 ft from the plate, but falls low at the plate. Keep the pitch either inside or outside corner -- if you allow it to be hittable, minimize the damage.
Then I introduce the pitcher to the various grips and releases that are widely used:
1. 4 finger/palm grip
2. Using index finger and thumb to make a circle
3. Grip that uses only the middle/long finger on the ball
4. 3 or 4 finger grip with a backhand flip release.
The first 3 are grip only variations. the release is the same: locked wrist/no snap; the ball comes off the palm and not rolled off the fingertips
The 4th is the most difficult to master but the effect is the most devastating - the ball comes to the plate looking hittable then seems to pop a parachute. It dies in speed and drops down across the plate.
Finally I tell the pitcher to try each grip and release, decide on one to concentrate on, and then practice it constantly. (Like shooting free throws in basketball -- establish a shot, develop a rhythm, stay with it, work on repetitions)