change between text message layouts

hmmm, i never noticed this option on the stock messaging app, but if bubble style threading is what you're after, try the handcent sms app from the market, download and install it and use it instead of the stock, just be sure and turn your stock messaging notifications OFF so you don't get double text notifications, but handcent will give you the bubbles, you can even customize the color of the bubbles, ringtones for different contacts, add a pop up to wake your phone up when you get a text, all kindsa cool options and the best part...handcent is free, hope this helps 

Are you sure it's not the "Messages" widget? If you go Personalize>Widget>Messages you'll be prompted to choose between two formats, one that's a list, and one that looks more like individual bubbles.