You have the option to skip upgrading NSX agents, but this is not recommended.
You must upgrade NSX agents before upgrading PCG, but you have the option to skip upgrading NSX agents as a feature to continue with the workflow under certain conditions. We do not recommend skipping the upgrade of NSX agents because VMs with NSX agents in a different version compared to PCG will lose connectivity with the PCG.
Why you may consider skipping agent upgrade:
You want to upgrade only selected private clouds within your public cloud.
You do not want any downtime on certain critical managed workload VMs.
You do not want powered off VMs to block the upgrade process.
You may want to apply a bug-fix patch only to the PCG without affecting the agents.
If you do skip upgrading NSX agents, you cannot upgrade them later using the UI or APIs. This can potentially break connectivity between such NSX agents and the upgraded PCG. The only workaround for restoring connectivity in that case is to uninstall the old NSX agents and reinstall the latest NSX agents on those VMs.
If there are any quarantined VMs, they are not included in the upgrade plan and you cannot upgrade them. Quarantined VMs do not have any connectivity with PCG and therefore not upgrading them does not affect the upgrade process.