Low-fare guarantee is only good if you find a lower fare within 24 hours (confirmed that with Web Services vs. "same" calendar day).
To OP... sorry, the market dictates the price, and you ended up paying a premium by buying earlier... what you get is availability. The longer you wait, the more risk, and price may change up OR down.
To answer your question... yes, it's policy to pay the change fee (dependent on your fare). So if the price difference is more than the change fee (whether it's $30 or $100) you get the difference.