Change grading scale after grades are issued?

Problem: Need to change the grading scale after students were graded but Moodle won't let me

Salient facts:

• Using Moodle 2.4 
• Class was converted from 1.9, so this could be a legacy issue.
• A manual assignment entry was added directly in the gradebook when class was taught in 1.9 last year
  
• The grading scale was set to be a maximum of 4 points
• Today the Instructor has issued grades for this assignment
  
• Oops, the scale should have been 3 points, not 4
  
• When I try to edit the entry directly from the gradebook, it will not let me change the 4.0 to a 3.0. It is uneditable. 
• There is no corresponding activity elsewhere in the class because remember, this was a manual entry created directly in the gradebook.
  
• Image may be NSFW.
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