Testing and QA

From the upgrade docs:

"Do not overwrite an old installation unless you know what you are doing ... sometimes old files can cause problems in new installations. The best way is to rename the current Moodle directory to something else, then unpack the new Moodle archive into the old location."

"Next, copy across your config.php, any other plugins such as custom themes, and your .htaccess file if you created one:"

This obviously implies that if you added things to Moodle or changed code in the past then you need to re-add them to the new installation (carefully, testing each one for compatibility).