Rewriting GET request parameters while configured as a reverse proxy

[Rv Rv]

Hello
Is there a way to make nginx rewrite the GET request parameters while configured as a reverse proxy. e.g. if nginx receives a request GET / foo.html?abc=123 , can nginx rewrite it to GET /foo.html?abc=456 (nginx admin specifies 123 to be changed to 456) and then do a proxy pass to the origin server. 
I did a test run with using $args on the lines of 
if($args~post=140){
rewrite ^ http://example.com/ permanent; }
as explained at http://wiki.nginx.org/HttpRewriteModule. However, this seems to work only for when nginx is the web server. It tries to fetch the content from the local nginx html folder.

Please provide inputs.  
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