public function init()
{
    parent::init();

    $oldVar = "";

    //Event: onBeforeSaveEntry
    craft()->on('entries.beforeSaveEntry', function (Event $event)
    {
        $entryModel = $event->params['entry'];
        $oldVar     = $entryModel->fieldVar;
    });

    //Event: onSaveEntry
    craft()->on('entries.saveEntry', function (Event $event)
    {
        $entryModel = $event->params['entry'];
        $newVar     = $entryModel->fieldVar;

        if ( $oldVar !== $newVar ){
            // ...
        }
    });
}

public function init()
{
    parent::init();

    $oldVar = "";

    //Event: onBeforeSaveEntry
    craft()->on('entries.beforeSaveEntry', function (Event $event) use (&$oldVar)
    {
        $entryModel = $event->params['entry'];
        $oldVar     = $entryModel->fieldVar;
    });

    //Event: onSaveEntry
    craft()->on('entries.saveEntry', function (Event $event) use (&$oldVar)
    {
        $entryModel = $event->params['entry'];
        $newVar     = $entryModel->fieldVar;

        if ( $oldVar !== $newVar ){
            // ...
        }
    });
}