In the past, this was done when installing/upgrading full istio, and involved some duplication - ports configured
both in upgrade, Gateway and VirtualService.

The new Ingress chart uses a 'values.yaml' (see user-example-ingress), which auto-generates Service ports,
Gateways and basic VirtualService. It is still possible to only configure the ports in Service, and do manual
config for the rest.

with the Service and bound to the namespace, it is recommended to:

1. Install the new gateway in a new namespace.
2. Copy any TLS certificate to the new namespace, and configure the domains.
3. Checking the new gateway work - for example by overriding the IP in /etc/hosts
4. Modify the DNS server to add the A record of the new namespace
5. Check traffic
6. Delete the A record corresponding to the gateway in istio-system