If I were you I'd add your old install to the new /boot/grub/menu.lst
Grub has a different format/syntax/whatever to lilo, but you should be able to figure it out. It starts counting from zero so what was 1, grub calls 0.
/dev/hda5 grub calls hd(0,4) and /dev/hdb4 grub calls hd(1,3)
Did you get that? The rest of the options are basically the same as lilo
For example, this is an entry from /etc/lilo.conf to boot Mandrake
append="devfs=mount hdc=ide-scsi acpi=off quiet"
And this is to boot the same thing in /boot/grub/menu.lst
kernel (hd0,5)/boot/vmlinuz root=/dev/hda6 quiet devfs=mount
hdc=ide-scsi acpi=off vga=788
I find grub works better than lilo. Keep both your installs. You may need a lifeboat to climb into again.