Rewriting GET request parameters while configured as a reverse proxy

Server NGINX
1

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 Module ngx_http_rewrite_module. 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.

2

It seems your syntax is obsolete. Have a look at
Converting rewrite rules where it is
explicitly written.
It is also explicitely wriiten on the wiki page you visited that the
resource is obsolete and that you should use
Module ngx_http_rewrite_module.

B. R.