I can't help but feel as though i'm missing something here but anyway...
It should allready be booting the older kernel!
To boot:
"title Red Hat Enterprise Linux ES (2.4.21-27.0.1.EL)
root (hd0,0)
kernel /boot/vmlinuz-2.4.21-27.0.1.EL ro root=LABEL=/
initrd /boot/initrd-2.4.21-27.0.1.EL.img"
You should set the line "default=1" to "default=0"
And to boot:
"title Red Hat Enterprise Linux ES (2.4.21-20.0.1.EL)
root (hd0,0)
kernel /boot/vmlinuz-2.4.21-20.0.1.EL ro root=LABEL=/
initrd /boot/initrd-2.4.21-20.0.1.EL.img"
It should be set to "default=1"... which it allready is!