According to the Netherlands arm of Hardware.info, while Kaby Lake-based processors will physically fit into the LGA-1151 socket of Z370 motherboards, they will fail to boot. Since their post, Guru3D asked around to various motherboard manufacturers, and they claim that Intel is only going to support 8th Generation processors with that chipset via, again, allegedly, a firmware lock-out.
Thankfully, it's not Chocolate Lake.
Image credit: The Red List
If this is true, then it might be possible for Intel to allow board vendors to release a new BIOS that supports these older processors. Guru3D even goes one step further and suggests that, just maybe, motherboard vendors might have been able to support Coffee Lake on Z270 as well, if Intel would let them. I’m… skeptical about that last part in particular, but, regardless, it looks like you won’t have an upgrade path, even though the socket is identical.
It’s also interesting to think about the issue that Hardware.info experienced: the boot failed on the GPU step. The prevailing interpretation is that everything up to that point is close enough that the BIOS didn’t even think to fail.
My interpretation of the step that booting failed, however, is wondering whether there’s something odd about the new graphics setup that made Intel pull support for Z270. Also, Intel usually supports two CPU generations with each chipset, so we had no real reason to believe that Skylake and Kaby Lake would carry over except for the stalling of process tech keeping us on 14nm so long.
Still, if older CPUs are incompatible with Z370, and for purely artificial reasons, then that’s kind-of pathetic. Maybe I’m odd, but I tend to buy a new motherboard with new CPUs anyway, but I can’t envision the number of people who flash BIOSes with their old CPU before upgrading to a new one is all that high, so it seems a little petty to nickel and dime the few that do, especially at a time that AMD can legitimately call them out for it.
There has to be a reason, right?