Well, if it is a single texture with all the fonts on it, and it's fetching part of the image for each character, then you couldn't really replace the font with anything except a higher resolution of the same thing, or at the very least a new font with the same proportions.
If each letter is its own texture, that might not be as much of an issue, but you'd still want to keep sizing intact to prevent text from overflowing the window (or does the engine determine the necessary size of the window on the spot rather than having pre-sized windows per textbox?)
Since I've never actually done any modding or looked at any of the game's data files or even read any tech documents, the best I can do is make educated guesses and deductions. Would somebody who actually knows something care to comment?