Settings upgrade on boolean attributes doesn't work if default value is On and the old value in 6.5 preferences is Off. As an example this happens in the Random Mix settings.

The problem is migration code that looks like this:
=====================
$prefs->migrate(1, sub {
        $prefs->set('newtracks', Slim::Utils::Prefs::OldPrefs->get('plugin_random_number_of_tracks')     || 10 );
        $prefs->set('oldtracks', Slim::Utils::Prefs::OldPrefs->get('plugin_random_number_of_old_tracks') || 0  );
        $prefs->set('continuous',  Slim::Utils::Prefs::OldPrefs->get('plugin_random_keep_adding_tracks')   || 1  );
        $prefs->set('exclude_genres', Slim::Utils::Prefs::OldPrefs->get('plugin_random_exclude_genres')  || [] );
        1;
});
=====================

The result is that if 'plugin_random_keep_adding_tracks' is 0 before the upgrade the code above will set 'continuous' to 1 after the upgrade. The 'plugin_random_number_of_old_tracks' property will work correctly since its default value is set to 0.

The Random Mix plugin is one place where migration code like this exist, but I suspect the same problem might also exist in other places.