Meike Klettke, Uni Rostock: XML schema evolution and incremental validation (Slides, in German)
XML documents and document schemas change over time, that’s just a fact. Compare to SQL databases: Data is updated all the time, schema changes happen, columns are added to tables etc.
Update languages for XML are far from being standardized, and evolution of XML schemas hasn’t been considered much.
Especially schema changes are hard because there’s instance data that must be updated in order to be valid according to the new schema.
[skip over hard-to-summarize details]
(The simpler alternative pushed by vendors is schema versioning. Have multiple versions of schemas, validate against all (must match one), and downstream tools like XSLT scripts must be able to cope with multiple versions.)
Oracle’s XML component supports a limited form of schema evolution. MS SQL Server supports schema versioning.