Old Central Instance node showed as "down" by MySAP Agent

Within a specific SAP system, the central instance is migrated to a new node, and the old one is disabled. Anyway ITM V6.x still detects this old instance to be present into the SAP system and it shows

the node in TEP workspaces, flagging it as "down".
Also, any situation monitoring the status of available SAP instances would fire for it.

Actually, the agent discovers all of the instances defined in a SAP system. So, if the old central instance node is only "turned off" within the SAP system (and not really removed), the agent will still discover it.

There is a way to list the instances defined in the SAP system which will be also shown in the TEP navigator tree:
1) logon to SAPGUI and run the RZ04 TCode
2) Click on the "Instances/operation modes" button, and some configured instances in the SAP R/3 System will be listed. If no instances are shown here, mySAP Agent retrieves the instances by default mode
3) To retrieve the instances using the default mode, run the SE37 TCode, and type TH_SERVER_LIST for the function module name,
This way, the list of default instances monitored by the mySAP Agent will be displayed. If the name of the old central instance node is listed here, there is no way to avoid mySap agent from
retrieving it.
You need to remove the node instance from the SAP system.
If you cannot remove the instance node from the SAP system, the only way to avoid the alerts is to modify the situations monitoring the instance status attribute and filtering out the unwanted instance name, so that situations will not fire for it.
For instance, you can add a condition where the situation fires only if "Instance Host Name" attribute is different from <old central instance hostname>.