Manually installed SCOM agents are not "Remotely Manageable"

In both SCOM 2007 R2 and SCOM 2012, manually installed agents show up as Not Remotely Manageable. This includes agents which are installed via an SCCM package.

When these agents are not Remotely Manageable, the following tasks cannot be performed on them:
1. Change Primary Management Server
2. Repair
3. Uninstall

To get these agents back to being Remotely managed:
1. Login to the SQL server which has the OperationsManager DB
2. Open SQL Server Management Studio
3. Expand databases
4. Select OperationsManager database
5. Click on New Query
6. Copy the following

UPDATE MT_HealthService 
SET IsManuallyInstalled=0 
WHERE IsManuallyInstalled=1


7. Click on Execute