DigitalOcean has a pretty good write up of what has changed
and
grep -rnw '/path/to/php/files/' -e "ereg"
Can help you find deprecated commands
But - old habits die hard. A small website with a couple dozen pages - no big deal to make sure everything works right. New websites and the host has PHP 7 on it? Sure - I'd use it. A big website with thousands of pages... potentially costing hundreds of programming hours to fix? That's a long term migration plan.
If it isn't broken - don't fix it :)