Change Label for field used in Woo Commerce

It gets its label using a localisation call, __('IBAN', 'woocommerce'), so you could always just intercept that and change the text:

/**
* filter translations, to replace some WooCommerce text with our own
* @param string $translation the translated text
* @param string $text the text before translation
* @param string $domain the gettext domain for translation
* @return string
*/
function wpse_77783_woo_bacs_ibn($translation, $text, $domain) {
    if ($domain == 'woocommerce') {
        switch ($text) {
            case 'IBAN':
                $translation = 'MICR';
                break;

            case 'BIC':
                $translation = 'IFSC';
                break;
        }
    }

    return $translation;
}

add_filter('gettext', 'wpse_77783_woo_bacs_ibn', 10, 3);