You can check the upgrade eligibility for iPhone upgrade program here:
https://secure1.store.apple.com/shop/eligibility/checkUpgradeEligibility
But when i checked mine (i purchased iPhone 6 S Plus last year using iPhone Upgrade program) and it says I am not enrolled yet; not sure if this is a glitch or a bug but hope it will not stop me to pre-order tomorrow night.
This worked for me, but it says "currently unavailable" underneath each full price, which makes me think there's no way to select an iPhone Upgrade program price online.