It is necessary to have the Management servers at the highest version than of the clients.
The SSC (Symantec System Center) Console will need to be the highest or the same version as that of the Symantec Server and the ones below that will need to be same or lesser in version.
As long as the hierarchy is maintained, you shouldn't have any problems.
If you upgrade to 10.1.9, the server will indeed be able to manage your 10.1.8 clients fine.
versions 10.1.5.5000 ~ 10.2.0, 10.1.6.6000 ~ 10.2.1,10.1.7.7000 ~ 10.2.2,10.1.8.8000 ~ 10.2.3, 10.1.9.9000 ~ 10.2.4