Don't get confused by what the Apple Store lists! These configs are not related with the ability to physically swap out the CPU, just what Apple wants to aim you to buy.
Now to the brass tacks! Apple has waffled on the 21.5" systems over the years. Some are upgradable some not (socketed Vs soldered)! So far the 27" are socketed but they could change things!
OK will I be able to upgrade the CPU later right? Yes, if you have one with a socketed logic board or you'll need to swap out the complete logic board.
BUT its a very big job! And you need to use the correct tools to remove the display. If you don't it could be a very expensive Opps! Lastly, if you do it while the system is still under AppleCare or if there was a extended warranty Apple may refuse to help you. So I would wait at least a year before attempting and you might want to review the IFIXIT guides here: iMac Intel 27" Retina 5K Display Logic Board Replacement to get a handle on whats required.