Bartosz Dziewonski writes:
You could export the set of pages, edit the resulting
XML file to your
liking, and import them back under new titles.
https://www.mediawiki.org/wiki/Help:Export
https://www.mediawiki.org/wiki/Manual:Importing_XML_dumps
Actually, you might not even need to edit the XML after exporting. On Special:Import,
simply use the option “Import to a namespace” (or if you’d rather use subpages, “Import as
subpages of the following page”).
DanB