This just came in and it offers some additional clarification.
Following up on yesterday’s story, it appears that the 2015 27-inch iMac still uses plug-in RAM, so you’re free to upgrade it on your own after purchase. And while Apple limits you to a maximum of 32GB, OWC has just announced that it will shortly offer upgrade kits for 48GB and 64GB of RAM.
The kits, however, won’t be cheap, the 48GB kit selling for US$729 while the 64GB kit will retail for US$1,195.
Sadly the new 4K 21.5-inch iMac uses soldered RAM, so unless you’re extremely handy with a soldering-iron, you’re stuck with 8GB or 16GB, depending on the configuration you buy.
Stay tuned for additional details as they become available.
Via 9to5Mac