Why not just copy the old kernel image, modules and initrd image somewhere by hand before you upgrade kernels. If we try to make this automated it isn't going to be kiss. I used to do this way back in the day by including the entire kernel version in the pkgver and giving the images longer names. It was possible to have concurrently installed kernels. Check out how some of the AUR kernels manage to be the same kernel version as the official without causing issues.
