If you want to migrate to utf8mb4, which is recommended, you'll need to follow the four steps in that link: 1) check the database configuration, 2) make any needed database configuration changes, 3) run Moodle's mysql_collation.php script, and 4) change 'dbcollation' in config.php.
If you try this and there are problems, you can post in the forums and we'll try to help.