How to reverse '-proposed' channel package upgrade?

Question

I have an Ubuntu GNOME 16.04 with GNOME 3.18 64-bit VM, and in that VM I needed to test something in the xenial-proposed channel so I activated it in the software-properties-gtk, ran sudo apt-get update && sudo apt-get dist-upgrade and installed all the upgrades from the xenial-proposed channel as I couldn't find a way to get it just to upgrade the package I needed.

Now however I wish to downgrade all the packaged upgraded to their xenial-proposed versions to their version in the stable/normal channels, but how can this be achieved? Can I use ppa-purge to do this? And if so, how?

If I just disable the xenial-proposed channel via the same means that I enabled it will the packaged installed and upgraded to their xenial-proposed versions start to receive updates again from the stable/normal channels once the version numbers there are higher than the installed or will I receive no updates at all then until I enable the xenial-proposed channel again?

Answer 1

Adapting http://www.webupd8.org/2013/11/how-to-downgrade-proposed-repository.html and https://wiki.ubuntu.com/Testing/EnableProposed, the packages installed from the xenial-proposed repository can be downgraded by doing the following:

1. Disable the xenial-proposed repository in software-properties-gtk as you have suggested.
2. In /etc/apt/preferences.d, create a file named 99-downgrade-proposed.

3. Copy and paste the following lines into the file:

   Package: *
   Pin: release a=xenial
   Pin-Priority: 1001
   
   Package: *
   Pin: release a=xenial-updates
   Pin-Priority: 1001
   
   Package: *
   Pin: release a=xenial-security
   Pin-Priority: 1001
   
   Package: *
   Pin: release a=xenial-backports
   Pin-Priority: 1001
   
   Package: *
   Pin: release a=xenial-proposed
   Pin-Priority: -1
   

4. Run sudo apt update followed by sudo apt full-upgrade.

5. Delete the file by running sudo rm /etc/apt/preferences.d/99-downgrade-proposed.