emerge – ERROR: 17.1 migration has not been performed – upgrade from 13 to 17.1

When upgrading to 17.1 profile you need to perform additional steps to get to a healthy Gentoo portage system using profile 17.1.
Here what we stumbled when we tried to upgrade from 13 to 17.1:

>>> Emerging (1 of 3) dev-libs/elfutils-0.177::gentoo
 * Fetching files in the background.
 * To view fetch progress, run in another terminal:
 * tail -f /var/log/emerge-fetch.log
 * elfutils-0.177.tar.bz2 BLAKE2B SHA512 size ;-) ...                                                                                                                [ ok ]
 * Please follow the instructions in the news item:
 * 2019-06-05-amd64-17-1-profiles-are-now-stable
 * or choose the 17.0 profile.
 * ERROR: dev-libs/elfutils-0.177::gentoo failed (setup phase):
 *   ERROR: 17.1 migration has not been performed!!
 * Call stack:
 *        ebuild.sh, line 591:  Called __source_all_bashrcs
 *        ebuild.sh, line 410:  Called __try_source '/usr/portage/profiles/default/linux/amd64/17.1/profile.bashrc'
 *        ebuild.sh, line 467:  Called __qa_source '/usr/portage/profiles/default/linux/amd64/17.1/profile.bashrc'
 *        ebuild.sh, line 112:  Called source '/usr/portage/profiles/default/linux/amd64/17.1/profile.bashrc'
 *   profile.bashrc, line   6:  Called die
 * The specific snippet of code:
 *              die "ERROR: 17.1 migration has not been performed!!"
 * If you need support, post the output of `emerge --info '=dev-libs/elfutils-0.177::gentoo'`,
 * the complete build log and the output of `emerge -pqv '=dev-libs/elfutils-0.177::gentoo'`.
 * The complete build log is located at '/var/tmp/portage/dev-libs/elfutils-0.177/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/dev-libs/elfutils-0.177/temp/die.env'.
 * Working directory: '/var/tmp/portage/dev-libs/elfutils-0.177/homedir'
 * S: '/var/tmp/portage/dev-libs/elfutils-0.177/work/elfutils-0.177'

As you can see there is news, which consists of 12 steps to perform a successful upgrade to profile 17.1.
You can read the news with (the news output is included at the end of this article, just for completeness):

eselect news read 28

Specific note for our upgrade – in fact, our Gentoo installation was a chroot and it had not been updated for three years. The profile was 13.0 and portage supported only EAPI 5, which is a real problem. The new portage tree (we installed the latest) has no profile 13.0 and our emerge was half broken. We have preliminary steps described in Failed to install sys-apps/portage – Called pkg_preinst – portage._compat_upgrade.default_locations

Because we upgrade from 13.0 to 17.1, first, we need to perform the 17.0 upgrade (check the original steps here – https://gentoo.org/support/news-items/2017-11-30-new-17-profiles.html)

STEP 1) Select the previous profile 17.0 temporarily.

srv ~ # eselect profile set default/linux/amd64/17.0

You won’t be able to build any GNU GCC and you cannot continue with the next step.

STEP 2) Upgrade to GNU GCC 6.4.0 or later.

Our current GNU GCC is really old – 4.9.3. The latest version is gcc-9.2.0-r1 and we are going to use it.
Keep on reading!