http://www.debian.org/releases/stable/i386/ch-preparing.en.html#s-linux-upgrade
I didn't write the HOWTO, but href="http://kmself.home.netcom.com/Linux/FAQs/DebianChrootInstall.html">
http://kmself.home.netcom.com/Linux/FAQs/DebianChrootInstall.html
is nice description on how to do it.