Arun Batchu Blog from Jan 14, 2009

I had to scrub some private data in a few thousands of xml documents stored in an eXist database.

I used this:
update value expr with exprSingle

So, for e.g.

let $ssn-ids := $collection//u:PersonSSNID[. != '888888888']
update value $ssn-ids with '888888888'

The filter expression makes the xquery repeatable and precise.

Look up the update extensions here: http://exist-db.org/update_ext.html#N101B9 for more details and other update mechanisms. Quite useful.