If you change the namespace of your schema:

• You can't validate existing instances.

• XSLT/XPath/etc. will have to change.

• Applications may have to change.

Rules for changing schemata:

• If you change is compatible with existing instances, consider whether to keep the namespace the same.

• If you change is incompatible with existing instances, you should change the namespace.

• Ultimately, a content/application should always be valid against the schema they were authored against.