Cannot change time constraint on project task.
Expected behavior as project task has children.
This behavior is seen as the project task in question (PRJTASK0228599) has child tasks. This field is set to read only so the dates do not become out of sync between the parent (PRJTASK0228599) and child records (7 associated child tasks). The business rule preventing this behavior is 'Abort change of time constraint - parent':
This is an out of box business rule that prevents this field from changing when a task has associated child records.
This field can manually be changed by temporarily updating/disabling this business rule, however, this is not advised as this business rule is put in place to keep the project dates in sync.