making old theme backward compatible with 2.7

Unfortunately the theme I am using is no longer listed here but I would like to continue using it. I was reading through the information (https://codex.www.ads-software.com/Migrating_Plugins_and_Themes_to_2.7/Enhanced_Comment_Display#Detecting_WordPress_2.7) about making it compatible with the new functions of 2.7 but I can’t figure out where to put the commands. It isn’t clear to me whether the following lines to detect 2.7 go in comments.php or elsewhere:

if ( function_exists(‘wp_list_comments’) ) :
// new comments.php stuff
else :
// old comments.php stuff
endif;