I have to upgrade my sites from PHP 5 to PHP 7 and it is going to be hell on earth to accomplish this because PHP 7 has not made migration easy. One fortunate thing is having Cpanel's MultiPHP which allows you to work with different PHP versions as needed. For instance you can use PHP 5.6 on one domain while using PHP 7.1 on another domain. This allows you to test how a site would function on PHP 7 before making the transition.
I recently tested one of our simple sites and immediately encountered errors. One of the first things I noticed is PHP 7 installed through Cpanel's MultiPHP does not know where to locate Mysqli. I wonder if keeping PHP 7 as default and then placing all domains as PHP 5.6 will help solve this issue?