If you've got the "dateslip" option enabled in the config file, then they will be reshuffled by reply date vs. post date. If you turn dateslip off, they will remain in order by the original post date.
Also note - any changes that you make to the theme file probably won't affect your own view. Changes to the theme file apply to anonymous users and new users from that point on. Existing users (including yourself) will need to adjust their display prefs in My Home. <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />
