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With a bit of a workaround, an upgrade is still possible. Here are the steps:

First create a copy of the sources.list, and then replace the regular localized archive links with "old-releases":

cp -a /etc/apt/sources.list /etc/apt/sources.list.old

sed -i -e 's/de.archive/old-releases/' /etc/apt/sources.list

It might not be "de.archive" in your sources.list, but rather another TLD. After that, edit the file and comment out the entries for "partner" and "security":

#deb http://archive.canonical.com/ubuntu yakkety partner

#deb http://security.ubuntu.com/ubuntu yakkety-security main restricted

Run "apt-get update", it should not bring any error about missing repositories or such.

Next step: tell the upgrader that the current and next version are still supported - that part is not true, but it is required to make the upgrade work. Edit the file /var/lib/update-manager/meta-release and change the entries for "yakkety" and "zesty".

Old:

Dist: yakkety
Name: Yakkety Yak
Version: 16.10
Date: Thu, 13 October 2016 16:10:00 UTC
Supported: 0
Description: This is the 16.10 release
Release-File: http://archive.ubuntu.com/ubuntu/dists/yakkety/Release
ReleaseNotes: http://changelogs.ubuntu.com/EOLReleaseAnnouncement
UpgradeTool: http://archive.ubuntu.com/ubuntu/dists/yakkety-updates/main/dist-upgrader-all/current/yakkety.tar.gz
UpgradeToolSignature: http://archive.ubuntu.com/ubuntu/dists/yakkety-updates/main/dist-upgrader-all/current/yakkety.tar.gz.gpg

Dist: zesty
Name: Zesty Zapus
Version: 17.04
Date: Thu, 13 April 2017 17:04:00 UTC
Supported: 0
Description: This is the 17.04 release
Release-File: http://archive.ubuntu.com/ubuntu/dists/zesty/Release
ReleaseNotes: http://changelogs.ubuntu.com/EOLReleaseAnnouncement
UpgradeTool: http://archive.ubuntu.com/ubuntu/dists/zesty-updates/main/dist-upgrader-all/current/zesty.tar.gz
UpgradeToolSignature: http://archive.ubuntu.com/ubuntu/dists/zesty-updates/main/dist-upgrader-all/current/zesty.tar.gz.gpg

New:

Dist: yakkety
Name: Yakkety Yak
Version: 16.10
Date: Thu, 13 October 2016 16:10:00 UTC
Supported: 1
Description: This is the 16.10 release
Release-File: http://old-releases.ubuntu.com/ubuntu/dists/yakkety/Release
ReleaseNotes: http://changelogs.ubuntu.com/EOLReleaseAnnouncement
UpgradeTool: http://old-releases.ubuntu.com/ubuntu/dists/yakkety-updates/main/dist-upgrader-all/current/yakkety.tar.gz
UpgradeToolSignature: http://old-releases.ubuntu.com/ubuntu/dists/yakkety-updates/main/dist-upgrader-all/current/yakkety.tar.gz.gpg

Dist: zesty
Name: Zesty Zapus
Version: 17.04
Date: Thu, 13 April 2017 17:04:00 UTC
Supported: 1
Description: This is the 17.04 release
Release-File: http://old-releases.ubuntu.com/ubuntu/dists/zesty/Release
ReleaseNotes: http://changelogs.ubuntu.com/EOLReleaseAnnouncement
UpgradeTool: http://old-releases.ubuntu.com/ubuntu/dists/zesty-updates/main/dist-upgrader-all/current/zesty.tar.gz
UpgradeToolSignature: http://old-releases.ubuntu.com/ubuntu/dists/zesty-updates/main/dist-upgrader-all/current/zesty.tar.gz.gpg

Basically "Supported" is set to "1", and the resource links are changed to "old-releases".

Run do-release-upgrade twice (first from yakkety to zesty, then from zesty to artful).

Afterwards enable the "partner" repository again, if it was enabled before.