The laptop I’m using had MySQL 5.1.55 installed with one database (apart from the system database). Installing MariaDB does not impact existing data in any way and once the install completed I had instant access to my data.
As part of the install you are given the option to set a new password for the root user. I choose to do it in the video, but you don’t need to. If you leave the password field blank the root password will not be changed. Other database users are preserved, of course.
As with any database upgrade, before doing this to a production system you should have backups and test.