grep -lr -e '<oldword>' * | xargs sed -i 's/<oldword>/<newword>/g'

This command broken down:

    * grep for the word in a files, use recursion (to find files in sub directories), and list only file matches
    * | xargs passes the results from the grep command to sed
    * sed -i uses a regular expression (regex) to evaluate the change: s (search) / search word / target word / g (global replace)
=========================


Perl provides a really nice one-line for this kind of thing:
perl -p -i -e ’s///g’ *

It also provides the option of creating a backup of each file changed:

perl -p -i.bak -e ’s///g’ *
http://www.linux.org/lessons/short/perlpie/perl_pie.html