Upgrade Path from 2.1.0

You can jump straight to 3.1.9, BUT you will need to go through each version's Release Notes and do each of the "Steps".

Obviously, make a complete backup before starting!

The first upgrade guide is the most important and has the largest set of steps. Pay particular attention to the Step 1 in those instructions. You can ignore Step 1 in all the subsequent upgrade guides because you have already installed the most recent "system" directory.

You will also want to use the following files from version 3.1.9 right from the start
index.php
config/mimes.php
config/constants.php

If the session library is using the database driver then do Step 2 of this guide when you get to Step 6 of Upgrading from 2.2.x to 3.0.x.

You are probably going to have to go through all the changes before even trying to run the upgraded site.

It has been a while since I've done an upgrade from v2 but I have done a few. It's not really that hard. Just be sure you understand each and every "Step".

In my mind Step 10 of Upgrading from 2.2.x to 3.0.x was the one that resulted in the most code rewriting.