Hi,
50x release onwards vCenter HA (AAM) agent is part of ESXi, it is not pushing from vCenter. The center server host name is resolving (both forward and reverse look up).
After the upgrade, all your inventory and other details are intact, which make sure DB is working fine.
Did you upgrade ESXi server, does it stateless ESXi or installed on Thin. Can you please check the /var/log/vmware/aam logs