Replace Query parameter value

You can use HTTP::query command to get or set the query string. You have to extract and rebuild it yourself then feed it to that command. In your case you want to remove devicename and add other items. Here is some example code to get you started...

when HTTP_REQUEST {
    set newquery "cmd=[URI::query [HTTP::uri] cmd]&"
    switch [URI::query [HTTP::uri] devicname] {
        router1 { append newquery "nodename=router1&newWindow=true&j_username=myuser&j_password=mypassword" }
    }
    HTTP::query $newquery
}