Yes, the agents can be a higher version than the server but not the other way around. If you ever decide to upgrade your v9 server installation you will also need to upgrade all of the agents to that version as well.
We recommend that you have the same version of Backup Exec and remote agent as the serial numbers are not compatible. If the remote machine is a workstation then you can install remote agent which do not require a license,in that case the remote agent would be compatible .