WordPress Change 'Featured Image' metabox

wordpress-hooks-metabox.php

<?php

/* Featured Image Metabox : change the text in the current metaboxes
/* -------------------------------------------------- */
add_filter( 'admin_post_thumbnail_html', function( $content ) {
	if(get_post_type() == 'post') {
		$content = str_replace('Uitgelichte afbeelding', 'Nieuws foto', $content);
		// $content .= '<p>Een extra uitlegtekst.</p>';
	} elseif(get_post_type() == 'page') {
		$content = str_replace('Uitgelichte afbeelding', 'Header foto', $content);
	}
	return $content;
});

/* Featured Image Metabox : change the title & position of the current metaboxes
/* -------------------------------------------------- */
add_action('do_meta_boxes', function() {
	// first remove the current metaboxes
	remove_meta_box( 'postimagediv', 'post', 'side' );
	remove_meta_box( 'postimagediv', 'page', 'side' );
    
	// add some new and improved metaboxes now
    // usage: https://developer.wordpress.org/reference/functions/add_meta_box/
	add_meta_box('postimagediv', __('Expertise icoon'), 'post_thumbnail_meta_box', 'post', 'side', 'high');
	add_meta_box('postimagediv', __('Case foto'), 'post_thumbnail_meta_box', 'page', 'side', 'high');
});

Can be done now by registering the custom posttype.