Starting in NSX-T Data Center 3.0.2, you can migrate an NSX Data Center for vSphere environment that uses a collapsed cluster.
In a collapsed cluster design, all management VMs, workload VMs, and optionally edges run on the same vSphere cluster that is prepared for NSX-v. The management VMs of the NSX-T Data Center must be initially attached to dvPortgroups. After migration, the management VMs of NSX-T Data Center will be attached to NSX-T VLAN segments.
The management VMs in the NSX-T Data Center include appliances such as NSX Manager, vCenter Server, VMware Identity Manager, and so on. The NSX-T VLAN segment ports to which these management VMs connect are blocked in two cases: when these management VMs are rebooted after they were migrated in-place by the Migration Coordinator, or when they are moved from NSX-V hosts to NSX-T hosts by vMotion in maintenance migration mode. Therefore, the management VMs might lose connectivity in such cases.
To prevent this problem, create a "management_vms" tag category, and add tags in this category. Assign a tag from this category to all the management VMs in the NSX-T Data Center environment. The migration coordinator migrates the VMs, which have tags in the "management_vms" tag category, always to use the unblocked VLAN segment ports.