Just a quick tip for anyone still trying that I forgot to mention:
When you load up the iOS Apple Store app, rather than tapping on the "Pre-order" link on the main page, scroll down a bit and tap "Check Eligibility Pricing." It'll ask for your phone #, billing zip code, and billing password if you have one. From there, provided you're eligible for an upgrade, it'll let you pick which phone you want.
That's how it ultimately ended up working for me, could've just been a lucky coincidence though.