Upgrading from EE 2.1.3 to 2.9.2 - PHP Errors abound

Hello knowledgeable people,

I am in the process of upgrading from 2.1.3 to 2.9.2 and have followed this guide https://ellislab.com/expressionengine/user-guide/installation/update.html only to find php errors and a non-functional site on the other end.

Would anyone have any insight on performing this upgrade? What are the gotchas and the good-to-knows?