Standard 27-inch iMacs have a small hatch in the back that allows the RAM in the machine to be upgraded after purchase, but the iMac Pro does not have that feature.

There is no rear hatch because the RAM in the iMac Pro is not user upgradeable following purchase, but there's good news -- an Apple Store or an Apple Authorized Service Provider is able to open up the iMac Pro and swap out the RAM.

iMore's Rene Ritchie spoke to Apple and learned that any service center is able to upgrade the RAM on an iMac Pro following purchase.

Yes. Any service center, Apple or indie, can upgrade the RAM on iMac Pro post-purchase. https://t.co/4JIRCSsu5H — Rene Ritchie (@reneritchie) December 14, 2017

At Apple Stores, iMac Pro users will likely only be able to upgrade to Apple-provided RAM, but third-party service providers will be able to offer non-Apple RAM and might even allow users to bring in their own RAM. Policy will undoubtedly vary by location, however.

The entry-level $4,999 iMac Pro ships with 32GB of 2666MHz ECC RAM, but up to 128GB RAM is supported. Apple charges $800 to upgrade to 64GB RAM and $2,400 to upgrade to 128GB RAM. Upgrading RAM after purchase in eligible machines is often more affordable than purchasing Apple's RAM upgrades at checkout.

There are no post-purchase options for upgrading the SSD, processor, or graphics card in the iMac Pro.

Apple began allowing customers to purchase the iMac Pro this morning. 8 and 10-core machines will begin shipping out this week, while 14 and 18-core options won't be available to ship out for 6 to 8 weeks. While the entry-level iMac Pro is priced at $4,999, there are a number of upgrades available. A maxed out 18-core machine with 4TB of storage and a Radeon Pro Vega 64 is priced at $13,199.