How do I change an incoming uri to a different string before the VIP sends it to the Pool?

just an example when you run tcpdump (i.e. ssldump is used to decode http traffic, so you can see http header).

 configuration

root@(ve11a)(cfg-sync Changes Pending)(Active)(/Common)(tmos) list ltm virtual bar
ltm virtual bar {
    destination 172.28.20.111:80
    ip-protocol tcp
    mask 255.255.255.255
    pool foo
    profiles {
        http { }
        tcp { }
    }
    rules {
        myrule
    }
    source 0.0.0.0/0
    source-address-translation {
        type automap
    }
    vs-index 2
}
root@(ve11a)(cfg-sync Changes Pending)(Active)(/Common)(tmos) list ltm pool foo
ltm pool foo {
    members {
        200.200.200.101:80 {
            address 200.200.200.101
        }
    }
}
root@(ve11a)(cfg-sync Changes Pending)(Active)(/Common)(tmos) list ltm rule myrule
ltm rule myrule {
    when HTTP_REQUEST {
  if { [HTTP::host] equals "server.domain.com" and [HTTP::uri] starts_with "/webapplication?frc=AO" } {
    HTTP::uri "/flexiblewebapplication/index.jsp?frc=A0"
  }
}
}

 packet trace

[root@ve11a:Active:Changes Pending] config  ssldump -Aed -nni 0.0 port 80
New TCP connection 1: 172.28.19.251(55281) <-> 172.28.20.111(80)
1378564869.6187 (0.0019)  C>S
---------------------------------------------------------------
GET /webapplication?frc=AO HTTP/1.1
User-Agent: curl/7.15.5 (i686-redhat-linux-gnu) libcurl/7.15.5 OpenSSL/0.9.8b zlib/1.2.3 libidn/0.6.5
Accept: */*
Host: server.domain.com

---------------------------------------------------------------

New TCP connection 2: 200.200.200.14(55281) <-> 200.200.200.101(80)
1378564869.6250 (0.0032)  C>S
---------------------------------------------------------------
GET /flexiblewebapplication/index.jsp?frc=A0 HTTP/1.1
User-Agent: curl/7.15.5 (i686-redhat-linux-gnu) libcurl/7.15.5 OpenSSL/0.9.8b zlib/1.2.3 libidn/0.6.5
Accept: */*
Host: server.domain.com

---------------------------------------------------------------