how to change the values of a task based on updating a parent issue

[Ankur Mehrotra]

1.Create a parent issue with pid field
2.Create a task (pid gets automatically populated) using post function
3.When we update the parent issue - pid field, the task pid doesn't gets updated ,please help for the same.

[Ankur Mehrotra]

any update from any1

[Ankur Mehrotra]

Let me frame my question again

1.I am Creating a parent issue which is having field name e.g pid 
2.I am Creating a sub-task under the above parent issue created and after that - the pid field in sub-task gets automatically populated ,which is done using post function 
3.My requirement is when we update the above parent issue of the pid field, the task pid field of the sub-task doesn't gets updated.I want it to get updated.
Please help for the same.

Can we somehow perform some action on the update of any issue,using listeners or any other way ?
If Yes how ?