Issue with replacing Set Cookie path within iRule

You could try something like this to iterate through the cookies present.

when HTTP_RESPONSE {
    set cookies [HTTP::cookie names]
    foreach cookie $cookies {
        set cookie_value [HTTP::cookie $cookie]
        set cookie_path [string map -nocase {"/OLD" "/NEW"} [HTTP::cookie path $cookie]]
        HTTP::cookie remove $cookie
        HTTP::cookie insert name $cookie value $cookie_value path $cookie_path
    }
}