prevent apt from downgrading packages

[Felix Rubio <felix@kngnt.org>]

Hi all,

    I have mix stable+testing (~1200 packages from stable, ~20 packages 
from testing). I have the following contents in 
/etc/apt/preferences.d/pinning:

Package: *
Pin: release a=stable
Pin-Priority: 1001

Package: *
Pin: release a=testing
Pin-Priority: 200

Package: *
Pin: release a=experimental
Pin-Priority: -10

Package: *
Pin: release a=unstable
Pin-Priority: -10

What happens is: after an apt update, I list the packages to be upgraded 
with apt list --upgradable, and I get only those that will be upgraded. 
However, when I run apt upgrade, I get a number of packages that will be 
downgraded. How can I prevent this downgrading from happening?

Thank you!
Felix