MySQLi shouldn't care if you use MySQLi or not. That is probably an issue with an upgrade to your php version. What happens when you try to use the standard MySQL connection?
It isn't as simple as updating the connection. All of the php code that refers to the database would have to be updated as well. The easiest solution is to move to a php version 6 or lower which should still support standard mysql connections. The more difficult solution is updating all of the CMS code, not just the connection, to mySQLi.