Underground Wizard Archetype and Favored Terrain

[chaoscowboy]

Hello, I'm back with more questions. I've decided to take a crack at converting some of Legendary Game's adventure path additions to Iron Gods, and there's a neat little wizard archetype that has some abilities that are kind of cool. It gets favored terrain (underground) and as a selectable ability every 3 levels, one of the options increases that bonus. Here's the issue I'm at.

I bootstrap favored terrain (underground) with an ability at the right level. Ok that's solid. It works. Then, I've fiddled with the script on the ability that increases the bonus. I've gone with the following:

hero.child[ftUndergro].field[abValue].value += 2

Problem is the little manual upgrade option still shows it as 2 rather than 4 once this ability has been upgraded once. The script says I'm unable to change abUser which is the other field that holds the value.