by Ken Task.
In each Moodle instance there is a config.php file:
$CFG->passwordsaltmain = 'bunchoflettersnumberscharacters';
Introduced in version 1.9.6 (if I re-call correctly) but system admin did have the option to ignore the repeated warnings (I think). So, you may not have a salt line in the config.php file of that 1.9.6 site.
If you do, however, do copy it to the 2.3 config.php but see and adjust accordingly: http://docs.moodle.org/24/en/Password_salting
'spirit of sharing', Ken