Once you have upgrade your DM server, the Agent files in the Repository will be replaced with the latest one.
The things I'm asking is to update the Agent on the Thin Client. Basically you need to drag the "update Agent" template to those devices with old Agent version, the task is mainly to download the latest Agent files from your Repository to the Thin Client and do the upgrade.
The newer Agent will provide new/enhanced functionality comparing with the old one, so please make sure the DM Agent on the Thin Client side is always up to date before sending other tasks.