Updating the Max Grade on a manual grade item does not change the range for the item on the user report.

To Replicate:

Create a course and enroll a user.
Create a manual grade item with all default settings and max grade of 50.
Edit grader report and give student a grade of 45 on the grade item.
Look at User report, see that Range is 0-50 and grade is 45.
Go back to Categories and Items and edit the grade item, changing max grade to 45 and save.
Go back to user report and see that Range is still 0-50 and grade is 45.