[Koha] Changing itemtypes?

We've got a new boss, well, kind off, and she got some things going. Now I have been asked to delete some itemtypes, which is no problem when they are empty. Is there a possibility to change itemtypes in the database without screwing up the whole system? (I tried something like UPDATE items SET itype = 'CAS' WHERE itype ='AAC'; and UPDATE biblioitems SET itemtype = 'CAS' WHERE itemtype ='AAC'; to take an example, but that did not really change anything, or rather, it did, but it wasn't working that well.) I figured that when I change the itype/itemtype in biblio/biblioitems it still does not change in biblio.marcxml, is there a way to do that? Thanks, Marty