If you really need 256GB of RAM inside your iMac Pro, you’d better make sure you buy it that way from Apple. It seems it is impossible for owners to upgrade to this configuration after purchasing their machines — even if they have the latest model.
Apple introduced the 256GB of RAM option to the iMac Pro last week. It’s a costly configuration, with the RAM alone priced at $5,200 on top of the iMac Pro’s base price of $4,999. But don’t think about trying to get a cheaper upgrade after you’ve received the new all-in-one.
New Apple documentation suggests there is no way for machines that didn’t ship with 256GB of RAM to be maxed-out later.
Read more at CultOfMac.com