You must upgrade EMS to 6.4.8 or later before upgrading FortiClient.
FortiClient EMS supports direct upgrade from EMS 6.2. To upgrade older EMS versions, follow the upgrade procedure that FortiClient and FortiClient EMS Upgrade Paths outlines.
With the new endpoint security improvement feature, there are backward compatibility issues to consider while planning upgrades. See Recommended upgrade path.
FortiClient EMS does not support downgrading to previous EMS versions.