If the 256 page document is the result of a mail merge, then if you have the original merge document, and its data source, make the change there and re-merge with
http://www.gmayor.com/individual_merge_letters.htm to create/recreate the separate documents.
You could process all the separated documents using
http://www.gmayor.com/document_batch_processes.htm
to replace the footer text with new footer text.
or you could use an includetext field in the footer of your large document to call the content of the document as required.
Much depends on where you can start from.