There were some things that really needed some adjustment, one of them was Fultz' upward momentum from his legs. Shooting is about rhythm, you don't want forces to counteract which will lead to misses when you're tired and it creates inconsistent motions.
This is a picture of Fultz' workout prior to SL. You can see the space between his heels and the ground increasing while he's bringing the ball up to his set point (1-3). He's at the maximum height at 3 and as a result he can't bring any momentum from his legs to the ball when he's actually getting the ball to his release point (3-4).

Compare that to Curry's form. You can see a gradual elongation of his knees and momentum from his forefoot throughout the entire motion and reaching the maximum verticality when he's releasing the ball.

You can see the improvement in the video from Unbreakable. Now he needs to get comfortable with it and do it more in control, he loses balance a bit because it's so explosive. No need for that with free throws and such a relatively short distance.
Another thing that's apparent is his set point (3 in image) than in his college season. The red lines and circle indicate the positions of his arms and ball when he shot it in college.

Personally I would like to change it back, since it makes it easier to release it with the proper angle without pushing the ball like he did during scrimmage. Once again the quoted video indicates this transition back to his former set point.
At last for those who were wondering, this is what Fultz' likely wanted to change during the offseason. Some players in the NBA have a so called "one-motion" shot. This is when you simultaneously move the ball up with your upward momentum and extend your elbow to speed up the release, illustrated with picture below.

Most players bring the ball up to their set point and the only movement left is the extension of the elbow. The one isn't better than the other, most people with two-motion shots can adjust better in the air and shoot over people like Damian Lillard, Klay Thompson, Kevin Durant, etc and the one-motion shot is quicker.
This is a personal preference and Fultz should just choose which one he's more comfortable with, he used a two-motion shot in college and high school and imo probably suits him better as a tough shot maker.