Hello 🙂 Is it possible in ktor clients to change default request url on the fly (after config) ?

No, but you can create a new client with overridden config:

val client = HttpClient(Apache) {
    defaultRequest { 
        url("<http://example.com>")
    }
}

val newClient = client.config {
    defaultRequest {
        url("<http://httpbin.org/get>")
    }
}