AMD has now clarified that it will enable owners of X470 and B550 chipset-based motherboards to upgrade to Zen 3. However, this upgrade would be an opt-in irreversible "Beta" BIOS upgrade that would remove compatibility with some of the older Ryzen processor in order to make space for new Ryzen 4000 Zen 3 Vermeer AGESA code.
https://www.notebookcheck.net/AMD-caves-in-to-demand-for-Zen-3-support-on-X470-and-B450-boards-with-an-optional-Beta-BIOS-here-s-all-you-need-to-know-about-AMD-s-proposed-upgrade-path.466045.0.html