Implement and support downgrade

[Teo]

Upgrading to 15.10 has been a catastrophe. Half an hour of use was enough to find like a dozen of critical regressions making the system unusable. Everything is broken, just look at my latest bug reports.

While you keep releasing major upgrades with so little testing and such a low quality assurance, you should make downgrading easy.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: ubuntu-release-upgrader-core 1:15.10.14.2
ProcVersionSignature: Ubuntu 4.2.0-23.28-generic 4.2.6
Uname: Linux 4.2.0-23-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Mon Jan 18 23:06:50 2016
InstallationDate: Installed on 2013-10-11 (829 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
Symptom: dist-upgrade
UpgradeStatus: Upgraded to wily on 2016-01-18 (0 days ago)
VarLogDistupgradeTermlog:

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Alberto Salvia Novella (es20490446e)]

Snappy Ubuntu already provides that, and eventually will become the default package management system:
https://developer.ubuntu.com/en/snappy/

[Teo (teo1978)]

But that will be when, in 2020??
In the meantime, everybody who upgraded to 15.10 is fucked, because it is completely unusable due to the astonishing number of critical regressions.

Either fixes for the huge number of critical regressions are released very soon, or a downgrade to 15.04 should be provided urgently.