Tbf it doesn't have a massive effect, I've a couple of.extra spells and a few are slightly easier to cast. Unless the difference is about twenty levels there's not then it's not that bad. The main imbalance will be that I've had access to more gold over a few games, so some of my soldiers are the better versions. I've seen some houserules to balance it slightly, like giving you more starting gold or something. I'll play either way. My band does have a couple of the new soldiers in though as we're playing both books.