Update JIRA subtask Fix Version when parent issue Fix Version updated

Update JIRA subtask Fix Version when parent issue Fix Version updated

When an issue is created, the Fix Version field is set to a particular
value (say 2.0). Then subtasks are created, and they inherit this value.
So far so good. But if later, the issue's Fix Version value is modified
(to say 1.0), the subtasks still keep the 2.0 fix version value.
Is there a plugin or technique I can use to keep these fields in sync?