This scenario is considerably different than a missing value stamp which makes a book incomplete. Also, this scenario is considerably different than what the perceived fraud originally discussed in this thread is which involves knowingly selling a comic that has been re-holdered in the wrong case.
I am not a conservator so I don't really have an opinion on that. However, I imagine there are many who think this is fraudulent while there are others that feel cleaning is okay. Once again, a completely different animal than swapping books in different cases.