Resolution: Won't Fix
Affects Version/s: 1.9.3
Fix Version/s: None
Steps to reproduce:
1. Set up a grade category in a course with a few existing grade items
2. Set the category's aggregation to Weighted Mean of grades
3. Edit the category's grade item (category total), and change its Grade Type to "None"
4. Edit the category again, change its aggregation to "Sum of Grades"
5. Edit the grade item: The grade type is now "frozen" to "None".
This "freezing" is not explained, nor does it make much sense.