I got a Genius Bar replacement once before from an iPhone 3G to a 3GS but only because the genius was incredibly rude. Keep in mind that I never requested a replacement from a 3G to a 3GS.
I needed an out-of-warranty replacement for my 3G, but the genius quoted me 4 weeks for a model to come in. He checked inventory at all other stores (I'm from the Silicon Valley) but no store had a compatible model available. He said that he was more than willing to replace my 16GB 3G with an 8GB 3G for the same price but there was none available within 30 miles. Keep in mind that this was during the time that the iPhone 5 was out, so I understood that there would be some sort of transit time, but this guy was just rude.
He acted like I was trying to get a replacement for free, said that if I did not want to pay money upfront that I shouldn't seek out a replacement, that all out of warranty genius service required some sort of payment. I slid my debit card across the counter to which he replied "since you have money, its easier for me to upgrade your line and cheaper for you". After that statement, I walked away.
I then got an email from Apple saying "How did your appointment go?", basically a survey, and I let them know what happened. The next afternoon, the manager from the store called me and apologized and invited me to come in and pick up a 16GB 3GS, available immediately, for the out-of-warranty price.