Debian lenny upgrade to squeeze

step 1.
$sudo vi /etc/apt/sources.list

deb http://ftp.tw.debian.org/debian/ squeeze main
deb-src http://ftp.tw.debian.org/debian/ squeeze main

deb http://security.debian.org/ squeeze/updates main
deb-src http://security.debian.org/ squeeze/updates main

deb http://volatile.debian.org/debian-volatile lenny/volatile main
deb-src http://volatile.debian.org/debian-volatile lenny/volatile main

step 2.
$sudo aptitude update
$sudo aptitude install apt dpkg aptitude
$sudo aptitude dist-upgrade

stop 3.
$sudo shutdown -r now
$cat /etc/debian_version or $lsb_release -a