As XML schemas evolve over time or as applications are integrated, it is sometimes necessary to validate an XML document known to conform to one schema with respect to another schema. More generally, XML documents known to conform to a schema may be modified, and then, require validation with respect to another schema. Recently, solutions have been proposed for incremental validation of XML documents. These solutions assume that the initial schema to which a document conforms and the final schema with which it must be validated after modifications are the same. Moreover, they assume that the input document may be preprocessed, which in certain situations, may be computationally and memory intensive. In this paper, we describe how knowledge of conformance to an XML Schema (or DTD) may be used to determine conformance to another XML Schema (or DTD) efficiently. We examine both the situation where an XML document is modified before it is to be revalidated and the situation where it is unmodified.