I think the upgrading part of MariaDB won't be such an issue, but you can download websites and run a local webserver like MAMP or WAMP to test your websites if they still work with the higher version of MariaDB
Higher load like that can also happen for many reasons, if you have run out of ram from too much load and your using swap. Or if linux kernel swappiness setting needs to be adjusted. Can also be caused by sql server configuration needing to be optimized to be better handle it's load.