Upgrading Lenny to Squeeze with backports involved

[Ignacio Vazquez Alvarez <ivazquez@s21sec.com>]

Hi!
This is may situation:
I'm using Lenny with Nagios3 installed from lenny-backports and now I'm 
thinking to upgrade the whole system to squeeze and I was wondering if 
there are special things to care about.
This is my sources.list:

deb http://apt-proxy.lab.cert.inteco.es:9999/security lenny/updates main 
contrib non-free
deb-src http://apt-proxy.lab.cert.inteco.es:9999/security lenny/updates 
main contrib non-free

deb http://backports.debian.org/debian-backports lenny-backports main

Because the Nagios3 version is the same both in lenny-backports  and 
squeeze (logically) I'm thinking in setting this sources.list:

deb http://ftp.se.debian.org/debian/ squeeze main contrib non-free
deb-src http://ftp.se.debian.org/debian/ squeeze main contrib non-free

and executing the update/dist-upgrade.

Can you confirm if this correct?

Thank you in advance. Kind regards.