59:  How do I perform a replace operation across more than one file?

  The "tags" feature of Emacs includes the command tags-query-replace which
  performs a query-replace across all the files mentioned in the TAGS file.
  See "Tags Search" in the on-line manual.

  In addition, Martin Boyer has written a package named global-replace
  which will perform a query-replace across all the files mentioned in the
  *compilation* buffer (usually done after a "grep"), which is available
  from:

    ftp://ireq-robot.hydro.qc.ca/pub/emacs/lisp/compile.el.z
    ftp://ireq-robot.hydro.qc.ca/pub/emacs/lisp/global-replace.el.z
    ftp://ireq-robot.hydro.qc.ca/pub/emacs/lisp/query.el.z

  NOTE: These files are compressed using GNU zip ("gzip"); you can get a
  copy from gzip from prep and its mirrors (see question 89).