mypmpro-change-checkout-selected-text.php

<?php

function mypmpro_change_membership_checkout_text( $translated_text, $text, $domain ) {
        
	switch ( $translated_text ) {
		case 'You have selected the <strong>%s</strong> membership level.':
			$translated_text = __( 'Thank you for selecting <strong>%s</strong>.', 'pmpro' );
			break;	
	}
	
	return $translated_text;

}

add_filter( 'gettext', 'mypmpro_change_membership_checkout_text', 20, 3 );