Documentation does not state compatibility of agents 6.9.0 - 6.9.3 with Windows versions earlier than Windows Server 2016.
Agents versions 6.9.0-6.9.3 are expected to work with Windows OS versions earlier supported by 6.8.*.
Updating agents in one go to the latest version is the preferred option. Upgrading to intermediate version first and then to the latest, will not provide any benefits. Nevertheless, very old versions (like 5.5) still might have difficulties to upgrade.