Can I modify guestbook.module safely? I want to make a few changes in the HTML output by the guestbook module.  For example, I want to enclose the submitters name in a &lt;div&gt; or a &lt;span&gt; (so I can style it better).

What's the best way to do this?  The only option I can think of is to modify the guestbook.module, but that could cause issues down the track.
Changing module code is usually a bad idea because it makes it hard to impossible to update to a newer version of the module.

Without knowing the structure of that module, changing the HTML/output should always be possible by overriding theme functions, which you can do without changing the original code.

See: http://drupal.org/node/11811