End-of-life vs. end-of-support vs. moving of repositories

Ubuntu 12.04 LTS had 5 years of supported life, so 5 years will not be reached until at least 17-April-2018 (https://wiki.ubuntu.com/Releases or https://lists.ubuntu.com/archives/ubuntu-announce/2014-April/000182.html) and many pages say it's not till end-of-month or 30-April-2019.

Many pages will tell you that when a release reaches EOL, after this date archives will be moved from archive.ubuntu.com to old-releases.ubuntu.com, it'll be country mirrors will drop the release, and mirrors also will drop it. If you look at https://launchpad.net/ubuntu/+archivemirrors you can get a glimpse of how up-to-date mirrors are, some are listed as 'last date unknown' (too long ago for counter) which means they could be much later in dropping the archives.

The best example of how fast it'll occur would be 12.04 LTS reaching EOL. It took months as I recall for it's archives to be moved, which is a huge contrast to 17.04 which took in comparison only hours. LTS releases are usually slower in moving, but without a set date you shouldn't rely on it.

It's easy to change your references to archive.ubuntu.com to old-releases.ubuntu.com (dropping any country code if you use it; eg. au.archive.ubuntu.com becomes old-releases.ubuntu.com too). If you are fully up-to-date before this move occurs though, I would expect a do-release-upgrade to be able to work anyway (I haven't tested it though), it gets far more complex if the next release (16.04 LTS) is EOL though.