when you disassemble the old seats, you will be able to inspect and see how bad the existing foam is. once it collapses, enough of the foam wears down or seat frame breaks through, the new covers will not look or feel as good. if you're not driving the car, now, you can begin teardown and inspecting even before the new covers arrive. The shape of the foam is pretty specialized.
One main point to replacing the seat covers is to make the interior look and feel nice. But if we skimp on the foam, and it looks lumpy or feels flimsy when sitting, we undermine our own efforts. So if the foam is coming apart, frames poking through, and not easily "patched" with local sources of foam, the reproduction stuff is the way to go.
We are blessed to have access to decent replacements from Legendary and the like until supplies/demand run out.