Well, that's a bummer. Just got off a chat with Apple Support and discovered that the newly announced iMac isn’t user upgradable.
Previously, this was the case with the 21.5-inch models. However, the bigger all-in-ones have always had user upgradeable RAM.
If you’re ordering a new iMac, it only comes with 8GB of RAM. Upgrading the memory will cost you an extra $200 for 16GB, $600 for 32GB, and $1,400 (!) for 64GB.
UPDATE: We found out that the customer support person was incorrect; the 27-inch iMac can be upgraded as usual. It's the iMac Pro appears that may be non-upgradeable. We apologize for the misunderstanding.