I was able to convert a PHP 5.4 app to 7.1 but after that there are too many backwards incompatible changes to be easy to fix. Even the upgrade to 7.1 took a few days of tweaks to the code.

In my case the upgrade was demanded by security because the latest PHP5 release is no longer maintained.

For an internal app you're much better off just leaving it on PHP 5. Maybe add an op-cache for performance.