On the Y-Series Coffee Lake SKU’s based on the reference boards, I would say there is backward compatibility with the Kaby Lake BGA. This also extends to the power requirements for the board so there should not be any power stability issues in theory if the OEM didn’t stray to far from the Kaby Lake reference design, hence no need to change anything in the design.
This will not be the case with Cannon Lake of course but for KL -> Coffee Lake, theoretically, it should be a direct upgrade at least for the Y-Series. Incidentally the Y-Series will also be featured in the Compute Card and the original reference designs which used KL now use CL with no changes to the board so far as I can tell.
Just as Kaby Lake supports DDR4 but is also compatible with DDR3, Coffee Lake (Y-Series at least) will be the same. There will be no need to upgrade the memory type just to support the new CPU. What the new CPU will mean however is the ability to address more memory which is the whole point of the upgrade from my perspective. Whether that is unlocked for all Y-Series SKU’s is yet to be seen but in theory it should be possible.
My primary interest in Coffee Lake is for increased RAM capacity support. If this cannot be attained in the new CL Y-Series then there is no benefit for me. The Y-series KL already has enough power for what I need and I’m especially impressed with the industrial design of your team as far as addressing Thermal management in this model. There has been minimal throttling under load from all reports which means you are able to squeeze max performance out of this CPU… that is quite impressive.
As for using the 32GB… actually I do quite frequently on my Aero, with my virtualisation work. While I appreciate this is not something the average user would do, for me, this is the reason I would buy the V. A small and portable form-factor that can still perform the power-hungry and memory intensive tasks I need, allowing me to leave the Aero behind for most short trips.