I know many people who were able to exchnage their 16GB for 32GB iPhones, no hassle, so I wouldn't be so certain.
When I bought my 3G and 3GS, I asked the Apple employee "What if I want the other color instead", both times I got the same answer, "You have 30 days to return if your not 100% satisfied with your iPhone."
I never did exchange it, I always kept what I got, but thats the answer I would get though.