Update of existing Ubuntu installation

[Rich schinnell]

I downloaded and created the cdrom for Ubuntu about 2 weeks ago and
installed it on my laptop.

A friend wanted a copy so I downloaded a new build dated 10/08/04 and I
burned that to a cdrom for him. (he is on dial-up so I felt sorry for
him :)  )

Can I just boot the new cdrom and have it upgrade my existing system
with any changes that were put in the newer system?  Or is there a
routine that I could use from the terminal prompt?

I get concerned that my system will not work when I try to install it
again and end  up with no email or preferences.

Inquiring minds need to be re-assured :)
Rich