I really doubt that mobo manufacturers will cannibalise their own sales by offering a new BIOS that enables PCIe4 on existing boards...at least not immediately. The only exception I see is on X470 if the X570s aren't ready at Ryzen 3000 launch; it'd be embarrassing for them to end up in that position though I think.
Though I suppose that if one manufacturer does it, the rest must follow.
A lot of them did supply a BIOS for Zen+.