how to replace kernel on Ubuntu

[loody]

Dear all:
I try to replace the kernel under my Ubuntu.
my questions are:
1. what is UUID used for? It seems replace "root=/dev/sda1"
    is there any tool or command I have to do to getting this number?
2. from 2.6, we use initramfs instead of initrd, but why we still need
to add "initrd" option in menu.lst.
3. is there any restriction of kernel version to replace in Ubuntu; if
I replace my ubuntu with older version as 2.6.18, will it be ok?
appreciate your help,
miloody