I think you have to be rooted to do so. Er, well, on my Thunderbolt, long before I rooted, I did find "themes" people had posted online that I could install, and some of them would change the color/ style of the bar itself, but the actual icons couldn't be changed. So, if you're just looking to change the bar, itself, try to see if there are any "themes" for whatever UI your phone has.