Change cell phones before 6 months?

I just bought my cell phone a week ago with SoftBank (2-year White Plan). However, I accidentally broke it, and I asked customer service if I could change to a new cell phone, and they told me that I could only change to the same model with my old cell phone. Now that I have no problem with that, I often hear from my Japanese friends that they want to change to new cell phone model before the 6-month period ends, but they can`t. I`m just very curious why is it that it`s so difficult to change to a new cell phone in Japan?
Thanks ahead!

The phone companies just don't want people to be signing up for a phone, then dropping that a few months later to go to just a slightly newer model - it's just a restriction they decided to put in place for their business consideration I guess :)