Figured it out. Found the correct syntax here: Link
 Code:
$ou = 'OU=MySubOu,OU=MyOU,DC=MyDomain,DC=com'
$user = MyUser
$password = MyPassword
$objNetwork = CreateObject("WScript.Network")
$strComputer = $objNetwork.ComputerName
$objComputer = GetObject("winmgmts:{impersonationLevel=Impersonate}!\\.\root\cimv2:Win32_ComputerSystem.Name='"+@wksta+"'")
$ReturnValue = $objComputer.UnJoinDomainOrWorkGroup("MyDomain\"+$user, $password, 0)
If $ReturnValue
	? "Failed removing "+@WKSTA+" from domain"
	? "ReturnValue = " + $ReturnValue
		GoTo END
EndIf
$ReturnValue = $objComputer.JoinDomainOrWorkGroup("Workgroup")
If $ReturnValue
	? "Failed joining "+@WKSTA+" to Workgroup"
	? "ReturnValue = " + $ReturnValue
		GoTo END