OK, je comprends. Il faut donc garder un œil sur les applications installées depuis les backports.
Comment vous comprenez ça (http://backports.debian.org/FAQ/) :
Q: What about dist-upgrade from lenny-backports to squeeze?
A: In general, backports.debian.org only has backports made from testing but with reduced version numbers (apt considers ~ lower than -), therefore the upgrade path from lenny-backports to squeeze is save.
En tout cas merci de la réponse !