You can wrap in something like:
<?php if( has_nav_menu('primary_main_links') ) : ?>
<?php wp_nav_menu( array('lots of parameter stuff here') ) ?>
<?php else: ?>
'other menu stuff here'
<?php endif; ?>
has_nav_menu() looks to see if a menu has been assigned by an admin to a menu region configured by yourself and if so will display it.
Or equally you could use the fallback parameter in the wp_nav_menu array to access a function and place all the older menu stuff in that within functions.php?