This is a list, the idea is that you add additional functions to this updater list and the loop later in the program runs them in sequence to upgrade the schema. So this line should be changed to

<pre>
updaters = [update_2xto30, update_30to31]
updaterVersion = [0, 300] #next entry is 300
</pre>

The logic is a little strange in my opinion though regarding the updaterVersion parameter. I made some changes in #417 pull request that cleans this logic up a bit but it'll likely be merged after your code so I wouldn't worry about this part. We will rebase #417 after we merge your code and update the logic.