As you've found, the
instanceType attribute is marked "system-only", since you could potentially mess with the replication state of the directory replica in which you modify this attribute.
You probably shouldn't be doing this!
To circumvent this protection, add the following registry value on a Domain Controller:
Name: "Allow System Only Change"
You could use PowerShell for this:
Set-ItemProperty HKLM:\System\CurrentControlSet\Services\NTDS\Parameters -Name "Allow System Only Change" -Value 1
Now you can modify the value of instanceType attributes on that DC. Either connect to that specific DC with LDP.exe/dsa.msc, or use the
-Server parameter with the Active Directory module cmdlets.
Remember to remove the registry key after making your changes.