Updating CodeIgniter 1.7.3 to CodeIgniter 3.1.7

CI3 is not necessarily backwards-compatible with CI2, and I suspect that much of your code will need to be modified, albeit not necessarily rewritten. Most affected would be the classes, IMHO, namely controllers, models and libraries, and then the configuration.

I suspect that the [upgrade from 2.2 to 3.0](https://www.codeigniter.com/user_guide/i...e_300.html) instructions would cover most of the changes needed. Take a look before despairing