Hi Tim, you really should make a subtheme if you are modifying things, it is very easy. you can rtfm (Read the fine manual) about subthemeing under
http://drupal.org/node/1010576

Although this is for zen, it basically applies to all themes, the basic steps:
  1. copy the theme folder that you want to modify
  2. modify the names to whatever you want it of
    1. the folder itself
    2. the themes .info file
    3. the themes css/theme.css (if exists)
  3. check that all the files are mentioned in your new theme .info and change it to be a subtheme of your main theme
  4. delete all the files you don't need to modify from your own subtheme folder
  5. that should be it.

In your case if you just changed node.tpl.php just remove everything but node.tpl.php and your subthemes .info file

Of course after that you have to install the theme in appearance section or by drush en yourtheme

It is more fun than it is fearsome.