We can override the schema declaration when setting xsi:nil attribute to value "true". The schema must allow this by setting attribute "nillable" to "true" (default value is "false").

There is no such mechanism in Relax NG.

We can change the type of the element by setting the value of "xsi:type" attribute. The new type must be validly derived from the overridden type. Validation Rule: Element Locally Valid (Element) .

There is no such mechanism in Relax NG.