Alder Lake-S will be the first to switch to LGA1700, meaning users will need to buy a new motherboard to upgrade. LGA1200 will only be used by the 14nm Comet Lake-S and Rocket Lake-S series. Alder Lake-S has PCI Express 4.0 support and there's now also confirmation at VideoCardz that the chip will have dual-channel DDR5 support.
AMD's Zen 4 is also expected to introduce DDR5 support and this new architecture will also require a new motherboard due to the adoption of a new Socket AM5.
Neither AMD nor INTEL have officially confirmed DDR5 support for their upcoming architectures. It is worth noting that both companies will shift to new platforms at the same time. AMD will adopt a new AM5 socket to support Zen4 architecture, while Intel will introduce LGA1700 socket for its (alleged) 600 motherboard series.