Upgrading from version 2.0.13 to 3.1.0 what needs to be modified。
You can see what has changed in the blog: https://blog.phalcon.io/
I would do it on staging server first, but if you are so brave to do it on production go ahead :)
Definately don't do it straight on production - or if you do, schedule downtime and create a snapshot to easily revert.
Upgrading from version 2.0.13 to 3.1.0 what needs to be modified
You need to recompile Phalcon. Also check your dependancies don't have a requirement of Phalcon 2 (usually in their composer.json file, or maybe README notes, but that's project specific)
For sure things can broke when changing from 2 to 3.1 straight away. Most likely model validation, maybe hasChanged() if you were doing it after saving models because there were a bug where snapshot was the same, even when saving model etc.