The newest version of Xampp comes with Maria. If I should update from the old Xampp to the newer version, I have MySQL databases. Are they readable and compatible with Maria?
Answer Answered by Ian Gilfillan in this comment.
You don't specify versions, but yes, it is usually very easy to upgrade, and in some cases even easier than to a newer MySQL version.
Take a look at MariaDB versus MySQL: Compatibility for details.