Hi, are are the vShield manager a two independent instances of you would upgrade & reconnect the vShield manager?
What features are you using from the vShield?
Depends on what features are you using in the vShield, you might need to rebuild the vShield objects in the new vCenter and reconnect the VMs to the new vShield objects.
Please note that vCNS is end of support and VMware recommends to upgrade to NSX.
If it's only a vShield Endpoint, you will get NSX for vShield Endpoint if you have vSphere Essential Plus and later edition
FAQ: Implementation of vShield Endpoint beyond EOA of vCNS (2110078) | VMware KB
"With the release of NSX 6.2.4, if you purchased vSphere with vShield Endpoint (Essential Plus and later), you can download NSX.
This means that NSX will appear on the vSphere download site, similar to vCNS."
End of Availability (EOA) of VMware vCloud Networking and Security (vCNS) in VMware vCloud Suite 6.0...
End of Availability and End of Support Life for vCloud Networking and Security 5.1 Standalone (20554...
http://www.vmware.com/go/lifecycle
Bayu Wibowo | VCIX6-DCV/NV
Author of VMware NSX Cookbook http://bit.ly/NSXCookbook
https://github.com/bayupw/PowerNSX-Scripts
https://nz.linkedin.com/in/bayupw | twitter @bayupw