For what it's worth, when my original iPhone was having some problems I was worried that they might replace it with the new 3G. I hated the 3G when it came out. In any case, i asked a store genius if my problems warranted a replacement would they give me the orig or the 3G, and they said at the time, "we always replace like with like".
The disclaimer: This was 4 years ago, and the orig had only been replaced by a month or so, and policies can change, but this is what I was told at the time. It would be easy to keep around some replacement stock of past models, but they may not want to deal with it.