I am thinking in 4c4 you can change the work center or machine up until the operation is in process. So if sequence 10 is the current operation, then you could change 20 or 30. But once sequence 10 is completed then you cannot change 20 when is becomes current. At least if I recall correctly - wait for someone who knows more about this than me.
My latest mantra - make sure you have latest stpapi patches and the latest session object. If on LN then please explore the option of using DAL2 functionality.
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