Check ChangeLog of FreeBSD port before upgrading

No upgrades are "mandatory" -- If the port version you're using works, no dependencies require the newer version, and there are no security holes you don't need to upgrade anything.

If you're asking about mandatory changes (e.g. new configuration files) as a result of a port upgrade, UPDATING is the right place to look, and there's also often something in the port's message file.
You may want to grep for your port's name rather than just doing a head -n 40 UPDATING -- If a lot of ports have UPDATING notes between the last time you did an update and now you want to be sure you're not missing anything.

Note that while port maintainers are usually on top of these changes sometimes we forget (or our test/use cases doesn't expose the change), so you should really be familiar with the underlying software and what changed between versions before installing any update.

For anything beyond the stuff above you would need to look at the changelog for each individual port, in whatever revision control system the original author/project uses. Remember that the ports tree doesn't "contain" software - It's just a collection of scripts that let you build someone else's software more conveniently.