Is upgrading MySQL to MariaDB seamless for Drupal sites
Drupal runs fine on both MariaDB and MySQL, so you should be able to replace one with the other without any issues at the application layer (Drupal).  I don't believe the abstraction layer does anything differently between the two so you shouldn't have issues with needing to change field types (swapping to Postgres or other engines Drupal supports would change that answer significantly).

I would expect with an update like that to require some downtime, and a change of username and passwords for the database itself. So you will probably need to update the database connection settings once the update is complete.

All that of course depends on WHM doing the actual update of the database layer correctly so that "test first" caveat is a big deal.