This article lists the OfficeScan versions that can be upgraded to Apex One 2019.
OfficeScan XG pre-SP1 communication is over HTTP, while OfficeScan XG SP1 and Apex One is over HTTPS. Verify if your environment's TLS configuration is uniform and current. For reference: Potential issues with HTTPS communication in OfficeScan XG Service Pack 1 and Apex One.
Once the server is upgraded via patch or Apex One, the connected agents will self-update to the same version unless configured otherwise. For reference: Controlling which agents will upgrade in OfficeScan/Apex One.
Please refer to the following for more information:
OfficeScan XG SP1
For a complete list of installation and upgrade requirements, refer to the Apex One 2019 product documentation.