I know that many upgrade their 27" iMacs, adding SSD with a normal hard drive. How easy is it to swap out the 500GB hard drive of the 21.5 base model with other sizes? I was hoping an iMac refresh would bump the base HD from 500 GB to 1TB. If there's no refresh soon, is it easy enough to replace the HD? I know the 21.5 doesnt have a room for a 2nd drive. just looking to replace the original. Thanks!