Client-Server communication via services is a 1 to 1 communication. When you create a client

auto client = node->create_client<ServiceT>(service_name, qos);

This client can only send requests to the service specified by service name. In order to change its service you need to create again the object.

From your question it looks like you want to send the same ServiceT::Request object to multiple servers. The ServiceT::Request object is not linked to any specific server or client and it can be reused as many times you want.

Assuming that you created multiple clients, what you can do is

ServiceT::Request request;

client1->send_request(request);
client2->send_request(request);