I had a hunch I was right yesterday when I researched the new iMacs from Apple for late 2015 – and I wanted to be sure before I posted otherwise, but the way I read the specs on Apple’s technical specs page for the iMacs was that only the 27 inch iMac was upgradable and site sponsor OWC confirms this with a teardown of the Apple’s 21 inch model.
Memory is soldered in. OWC recommends buying as much memory as you can at the time of purchase to increase the longevity and usefulness of your iMac.
This means that like OWC suggests that if you’re going to want to max out the ram on the 21.5 inch iMacs to full capacity you have to do so at time of purchase.
Also, check out the gallery at the link above as they have some great tear down and unboxing photos of the 21.5 inch retina iMac (Late 2015)