Right now we cannot create any UDN in already existing namespaces. Assigning a Namespace with 100 VM's to a new udn network or creating a UDN will not be accepted by OVN-Kubernetes. How I'm gonna explain that to a customer with hundreds/thousands of VM's in a single namespace which is willing to use UDN on existing Projects?

You cannot create UDN in already existing namespace. The annotation or label is immutable once the namespace is created. UDN was also meant to be introduced as an add-on to existing deployments. What is the intent behind "RIP and replace" approach - why can we not just leverage UDN in existing projects?

 
It should have been like: * create an additional network in a NS

• declare it as the primary network - through labels and annotation to namespace
• A restart of the pod / vm would bring them to the new network

https://github.com/openshift/enhancements/blob/master/enhancements/network/user-defined-network-segmentation.md