we upgraded a 100k lines PHP 5 codebase to PHP 7.1 in a month. if your php 5 code wasn't using any deprecated extensions (like the old mysql extension) upgrading is really easy. there are static analysis tools that spot the potential problematic code so you don't need to do it by hand. from that point on if you got any automation testing or unit tests you can have a good level of confidence that your upgrade was a success.