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:
Key: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\NTDS\Parameters
Name: "Allow System Only Change"
Type: DWORD
Value: 0x1
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.