Unable to update a forum activity due to scale type change

Prerequisites: Create a scale ABCDEF or anything else is ok. Course with some students enrolled. Non-rating activity: Create an assignment and set its grading options to use that ABCDEF scale. Go to the assignment grading interface and grade a student. Edit the assignment settings. Go to the "Grade" fieldset. TEST: Verify the "Some grades have already been awarded..." message is shown. TEST: Verify both the grade type and scale are disabled and with value "scale" and "ABCDEF" respectively. Save the settings. Edit the assignment settings again. TEST: Verify both the grade type and scale are disabled and with value "scale" and "ABCDEF" respectively. Rating activity: Create a forum and set its grading options to use that ABCDEF scale. Log in as student and create some discussion/post. Back as teacher (or admin), rate that post. Edit the forum settings. Go to the "Rating" fieldset. TEST: Verify the "Some grades have already been awarded..." message is shown. TEST: Verify the "When selecting a ratings aggregate type you must also select..." message is NOT shown. TEST: Verify both the rating type and scale are disabled and with value "scale" and "ABCDEF" respectively. Save the settings. TEST: Verify the "When selecting a ratings aggregate type you must also select..." message is NOT shown. And the form is saved ok. Edit the forum settings again. Go to the "Rating" fieldset. TEST: Verify both the rating type and scale are disabled and with value "scale" and "ABCDEF" respectively. Alternatively: run the lib/form/tests/behat/modgrade_validation.feature feature. It covers exactly the steps above.