From what is written in the Apple Store page for iMac 21.5 and 27" models, the 21.5 can be equipped with a 1TB Fusion drive, or 256GB flash, or 512GB flash, as options. There are more BTO options for the 27" model.
Not sure what an Apple Store would charge you to upgrade a new iMac to one of those, but you could call ahead and set an appointment with a product specialist, a Genius, to discuss the matter. If they can do it, then you'd probably arrange to take the computer in so someone could do the job and not interfer with the AppleCare plan coverage. Authorized persons should be the only ones to open up and upgrade; however the BTO models in the 21.5" would be the normal path to get that upgrade. Just like RAM, there is no user access to upgrade. (Built-to-Order ships from an Apple Store online.)
So that'd be my suggestion, ask them directly at an official Apple Store or see if Apple Support online http://www.apple.com/support/contact/ can answer the question for you, to see if the product can be upgraded by an Apple tech in a store near you.
Good luck & happy computing!