Hello,
I am not from Alfresco Engineering but as a partner have a decent amount of insight into the architecture / code of Alfresco. It is indeed the case that the 'sys:incomplete' aspect is not applied if you "only" change the type of a node. The relevant policy (IncompleteNodeTagger) is only triggered by node creation, property updates, aspect updates, creation and deletion of (child) associations. Unfortunately, changing the type is - as far as I see at the moment - the only change to an object that does not trigger one of these policies (apart from deletion, which is irrelevant for this use case).
I suppose this is not an intentional lack of policy coverage and personally would classify it as a bug against the documentation. Suggest you raise a JIRA to have that clarified.
Regards
Axel