To improve the quality of the morph, you can add shape hints to each keyframe. Shape hints let you tell Flash precisely which points in each vector to match up during the tween. So, if you add shape hints matching the corners of the square with the secondary anchor points of the circle, the morph will no longer appear to rotate. Here's how:
- Select the first keyframe and choose Modify Shape Add Shape Hint (Ctrl/Cmd-Shift-H).
- A small circle with an a inside will appear in the center of your shape. Drag it to the upper-left corner of the square.
- Select the last keyframe, and you will see a corresponding a shape hint. Drag it to the upper-left arc of the circle.
Note: There is a limit of 26 shape hints for each shape tween.