If a schema S1 is already referred to by another schema S2, schema S1 cannot be replaced by a new schema that has a modified file name or a modified namespace. This restriction does not apply if the create version option is used.

When you use the Attach button, you can only attach a schema file to the most recent version of an XML Schema asset.

If you attach a schema with a different file name and/or a different namespace, then there must not be another existing schema with same name and namespace.