Hi I am having this problem now too. I previously installed 7.0 on the domain as a trial and am now using 7.5, but the machines on the domain will not upgrade as they already have an agent (though they still don't show up in the asset list).
In order to deploy SysAid agents you can use the deployment tool.
Note that you will probably need to undeploy the agents first and then redeploy them with the newest version.
Please note this is not the only way to do it.
You can go to the SysAid interface for this task.
Please go to : Asset management > Network Discovery > Upgrade Agents Version.
Create a task to upgrade the agents to the latest version.
You can even divide the upgrade to groups to avoid traffic overload.
Please let us know if this solution solve your issue.