I need to downgrade some apt packages, that I had previously pinned to testing, having stable as default. This is the preferences file:

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

Package: *
Pin: release a=testing
Pin-Priority: 500

Package: *
Pin: release a=unstable
Pin-Priority: 400

When I check the policy for one of the upgraded packages, I get:

apt-cache policy libstdc++5
libstdc++5:
  Installed: 1:3.3.6-28
  Candidate: 1:3.3.6-28
  Version table:
 *** 1:3.3.6-28 0
        500 http://mirror.hetzner.de/debian/packages/ testing/main amd64 Packages
        400 http://mirror.hetzner.de/debian/packages/ unstable/main amd64 Packages
        100 /var/lib/dpkg/status
     1:3.3.6-27.2 0
        990 http://mirror.hetzner.de/debian/packages/ stable/main amd64 Packages
        990 http://cdn.debian.net/debian/ stable/main amd64 Packages

Whenever I change the settings for testing and unstable, I see the changes in the priority reflected here. Though, the priority for the stable package won't change, whatever I tried so far. The idea is to set it >1000, to actually execute the downgrade. Any hints, how to actually change it?