Cannot change time constraint on project task.
Expected behavior when a project task has children.
This behavior is seen when the project task in question has child tasks. This field is set to read-only so the dates do not become out of sync between the parent and child records. The business rule preventing this behavior is 'Abort change of time constraint - parent': /nav_to.do?uri=sys_script.do?sys_id=38183de79f333200598a5bb0657fcf4a
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.