Using plan.xml to update filter-mapping

Is it possible to modify a web.xml filter-mapping using plan.xml, more specifically, a to replace all current url-pattern with a new set of url-patterns? 

I keep on getting the typical Unmarshaller failed.

Or have I missed part of the manual which says only simple values in <variable-definition><value> element ?

Thanks for  sharing your experience.

F

Update : To better document what I'm trying to achieve : code snippets :

"AS-IS" : deployed web.xml contains filter-mapping 'myFilter' of which I'd like to change the <url-pattern>s.

<web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance