Congratulations, I think it will definitely be worth it. :) Keep me posted on anything you think is interesting and want to talk about.
I would just be guessing to answer your question, but I suppose it could be a number of reasons. It could represent the characters growing older, if not physically (MOMO, Jr, KOS-MOS, Ziggy) then mentally maturing as well. It may also be the directors wanted to shift the art direction, maybe to just test the waters or experiment. I've heard Xenosaga was just supposed to be a template for Xenogears, but I've never looked into that to see if it's true. I like to think of Xenosaga as its own series. Xenosaga II's art style was a bit less appealing to most people, but I can still appreciate it. It's like watching a pet or a child grow up. They start out cute and fun and then they go through an awkward phase, perhaps, and then they've fully matured into something great. I think that best represents Xenosaga I, II, and III. What do you think?