I thought about that, but I'm worried about ending up with a lemon. Doesn't something have to be wrong to do an exchange?
Yeah, technically. Find some asinine, niggling little problem with your phone (which is something that many MacRumors patrons seem to have little problem doing, heh), and if you goad Apple enough, they'll probably break down and replace it for you.
If you call Apple up and pay them for an Advance Replacement (where they put a hold on your credit card for the entire unsubsidized price of your phone model and ship you the replacement phone FIRST before you ship them yours back, and charge you $29 for the privilege), chances are that because there is nothing wrong with your old phone, and you paid them for an "extra-warranty" service, there is "no harm, no foul" and they're most likely not going to hassle you to reverse the exchange.
In neither case are you actually guaranteed an old-bootrom phone. Who knows: by the time you read this, Apple may have run out of stock on refurbished phones with the old ROM. (Chances are probably better finding one at an Apple Store, actually.)
Really, if you want a guaranteed swap, find one on eBay, buy that, and then sell your original phone on eBay. You might not break even, and so end up paying a few dollars for the privilege of having the old bootrom, but it's (probably) worth it.
-- Nathan