OK after a few hours of research I’ve used the WP Migrate DB plugin:
I’ve removed the two default Find and Replace rules and unchecked Replace GUIDs in advanced options.
After the conversion you will get a sql file with the converted data and structure so that tables that were on utf8mb4 are now utf8 (of course you have to import that file into the database – DO NOT FORGET TO DO A BACKUP OF THE EXISTING DATABASE FIRST).
You also have to change the setting in the wp-config.php file to define(‘DB_CHARSET’, ‘utf8’);
