It’s still early to know all of the internal details as iFixit hasn’t released a Teardown on these systems yet. Us independents won’t know for at least a year from now when we start getting them in for repairs & upgrades.
If Apple follows the same direction as the previous version the i3 will likely have a soldered CPU where as the i5 & i7 models will have a socketed CPU. Even still you are already buying the top model so at this point I don’t see any point on worrying about upgrades as the systems firmware will likely prohibit anything other than the supplied CPU’s this series support (iMac19,2). While not easy your systems RAM can be upgraded upto 32 GB. Your storage is locked in as both the SATA & PCIe ports are in use with a Fusion Drive set (HDD & SSD). The blade SSD in a Fusion config is only 32 GB which is quite small if you decide to use it at a boot drive. You’ll want to increase it to 512 GB or more so it can support the OS, your Apps and have the rest of the drive free for caching, paging and scratch space.