Advice on upgrade legacy app from 1.7.0?

Hi all,

I have inherited an old application created on CI 1.7.0 which I have been able to get running on PHP 7.1.18.
(It hits a fatal error on PHP 7.2.6!)

Ideally I would like to upgrade it to CI 3.x and I am seeking some general ideas on the best way forward.

My original plan was to incrementally upgrade the existing application through the releases but I am not 100% sure what code is CI framework and what code is application so overlaying the release tends to wipe out the application.

Would you advise trying from the other end? That is, install the latest 3.x with PHP 7.2.6 and attempt to port the application code to that?

I have a lot to learn so will welcome any advice you might share.

Thank you,
 - jon