Something does not work with changed zone stamps. You should somehow be able to get what you want after checking Options>Element Attributes>Zone Categories settings is pointing at the right stamp, and then selecting all zones and checking their settings, perhaps changing their categories back and forth.
But even then nothing is final. I just had to merge the file that had put me through all that into a new file, used attribute manager to transfer zone categories first, and I am finding I will need to redo the back-and-forth reminding operation again.
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http://archicad-talk.graphisoft.com/viewtopic.php?t=18618&highlight=