Please find the possible scenarios when the machines are renamed with _old

Scenario 1:   

When the workstations are swapped in the network. 

Consider two workstations, MACHINE-A with service tag ST-1 and MACHINE-B with ST-2. Both the workstations are swapped in the network. On swapping the workstations configurations is, MACHINE-A with ST-2 and MACHINE-B with ST-1. If you scan MACHINE-A, then the workstation MACHINE-B will be overwritten as MACHINE-A and the workstation MACHINE-A will be renamed as MACHINE-A_old. If you scan MACHINE-B again, it will overwrite the MACHINE-A_old as MACHINE-B.

Scenario 2: 

The workstation is already discovered with a valid Service Tag but scanned again with a different service tag. In this case ServiceDesk Plus will rename the existing workstation with '_old' and add the machine again with the new Service Tag.

Case 1:

In certain cases, the workstations might be discovered with valid service tag. But during its life cycle, you might change the motherboard of the workstation, thus changing the service tag. In this case, the existing workstation is renamed with _old and the workstation with the new service tag is added again.

Case 2:

The workstation MACHINE-A with service tag ST-1 is already scanned in the network. Another workstation, MACHINE-A with service tag ST-2 is added to the network. If you scan the workstation MACHINE-A again, it will rename the workstation as MACHINE-A_old and will add the newly added workstation as MACHINE-A.

Case 3:

In case of VMware machines, when the host is changed, the Service Tag gets changed too.

  Workstations overwritten with _old during Agent Based Scan:

If the agent is deployed through an 'Image', then the same Agent ID will be copied to all the client machines. This in turn, will create duplicate entries for the workstations when scanned and subsequently appends the previous workstation with _old. To overcome this issue, we request you to uninstall the agent completely from the registry and deploy it through AD GPO.