[Zope] Delete the properties and propertysheet defined by ZClass

Hi, all On my migration of Zope system, the one of ZClass common propertysheets has stopped using. So, first I just simply delete the definition of this propertysheet on ZClass, but namespace of existing ZClass instances seem to be polluted by old properties which were defined by that propertysheet. Second, get back that propertysheet definition using undo, then I try to delete the properties and propertysheet on each ZClass instance using manage_delProperties() and delPropertySheet(). but it was failed. Does anybody have any idea to clean up properties and propertysheet on ZClass instance? The version of Zope is 2.5.1 . Thanks in advance. --- kaz