Check if an Advanced Custom Field (ACF field) value changed.

functions.php

<?php

// Remember to insert the {$field_name}
add_filter('acf/update_value/name={$field_name}', 'yanco_check_change_of_field_value', 10, 3);

function yanco_check_change_of_field_value( $value, $post_id, $field) {

	$old_value = get_post_meta($post_id, $field['name'], true);
	
	if ($old_value != $value) { 
		// it changed 
	}

	return $value;
}