OWC has prepared RAM upgrades for the 27-inch 2019 iMac already - NotebookCheck.net News
2.0GB Memory Kit for 2006 - 2009 Apple Laptops and Desktops
OWC 16GB (2x8GB) ram Upgrade PC3-12800 DDR3L 1600MHz sodimm 204 Pin CL11 Memory Upgrade Kit Compatible with Late iMac, Mac Mini, and MacBook Pro Computer at Amazon.com