Migration failed : illegal network configuration: cluster subnet 100.64.0.0/14; overlaps built-in join subnet 100.64.0.0/16 in OpenShift Container Platform 4

Issue

• While migrating OpenShift 4.6 from OpenShiftSDN to OVN-Kubernetes, migration failed with an error:

  Migration failed : illegal network configuration: cluster subnet "100.64.0.0/14" overlaps built-in join subnet "100.64.0.0/16"
  

• How to change the OVN-Kubernetes internal join range of 100.64.0.0/16 if that is overlapping and already in use on the OCP cluster?

• Migration from OpenShiftSDN to OVN-Kubernetes is blocked because the join subnet of OVN-Kubernetes can not be changed.