To rename Subscriptions to Payment Plans on your My Account page, insert the following snippet into your functions.php file:

function start_modify_html(){
	ob_start();
}


function end_modify_html(){
	$html = ob_get_clean();
	$html = str_replace( 'Payment Plan', 'Subscription', $html );
	echo $html;
}


add_action( 'wp_head', 'start_modify_html');
add_action( 'wp_footer', 'end_modify_html');