you missed the 3rd sentence: You cannot cancel transfer orders created from posting change notices.
If you don't want process the posting change notice then you can set the status to completed in LU02.
But WM is just a sub to IM, and the 309 is completed in IM, so I do not really understand why the PCN should not be completed, you can always cancel the 309 with a 310 and this will create another PCN to undo what you did before.