Anonymous Monk,
While you have several replies that address changing 2003 to 2004 universally in a bunch of files, no one has mentioned that "rendered" text often does not look like the underlying HTML. Many tags could be "imbedded" in the text that you do not see as an end product. The methods provided so far do not address this. You will need to use one of the numerous HTML parsers such as HTML::TokeParser::Simple available on CPAN to get those cases. The problem is keeping track of the tags so the formatting is not lost either.