We'll need to wait for the IFIXIT teardown to understand more fully what is needed to replace or upgrade the RAM. I'm sure you have other questions on the SSD and general serviceability of this system. This system does not have an access door like the other 27" models.
Per Apple's notes: "The RAM can be serviced by a qualified Apple technician".
Now consider when you plan to add the RAM. Most people won't need more than 32 GB. Which gets into the systems warranty and if you void it by doing it your self, I would hold off until the warranty has expired. If you are doing this sooner, have Apple or Apples certified service techs do it so you don't loose coverage. Given the price of this system I would caution you doing this your self as it could be a very expensive Oops!