From an article in Forbes Jan 7, 2018
New CPUs will be backward-compatible with older AM4 motherboards, so long as they receive a BIOS update that will be available at the time - this is essential as AMD says its likely older boards will simply not work with new CPUs otherwise so you'll need to make sure you buy a board that's already had the update (these should be labelled as being AMD Ryzen 2000 ready) or are able to update the board yourself.
Perhaps the computer shop where you buy the new CPU will be able to update the BIOS on your motherboard for you.