Major-changes: Document the switch to OCI for updates

With the f42 rebase we will switch disk images to use OCI for
updates. While this is transparent for most users, it's still a big
technical change so it's worth mentionning it, at least so users using
can look into it.

See https://fedoraproject.org/wiki/Changes/CoreOSOstree2OCIUpdates
coreos/fedora-coreos-tracker#1823

Author: jbtrystram

modules/ROOT/pages/major-changes.adoc

@@ -9,6 +9,38 @@ This list is in reverse chronological order to keep recent changes at the top.
 
 // To add a new change here, see the template at the end of the file.
 
+== Switch to OCI updates
+
+Starting in Fedora 42, Fedora CoreOS will be updating through OCI images instead of the OSTree repo.
+Fedora Change request: https://fedoraproject.org/wiki/Changes/CoreOSOstree2OCIUpdates
+More discussion: https://github.com/coreos/fedora-coreos-tracker/issues/1823
+
+=== Planning
+
+This change has been introduced first in new disk images for the `next` stream, as part of the Fedora 42 rebase.
+The `testing` and `next` streams will follow when they rebase to Fedora 42.
+After a few releases we will migrate existing machines through a barrier release.
+
+|===
+|Update Stream |Release date
+
+|`next` 42.20250316.1.0 | 2025-03-18
+|`testing`| TBD
+|`stable`| Will follow `testing` as usual
+|===
+
+=== Notes
+
+Currently, Fedora CoreOS hosts pull updates from the OSTree repository.
+With this change, the hosts will pull updates from the Quay.io container registry instead.
+This should be a transparent change, altough proxied environnements require attention as the nodes will reach to a different address for updates.
+
+Note: Disk images will be updated first, so new installations of Fedora CoreOS based on Fedora 42 will use OCI images.
+After a few releases, we will migrate existing nodes.
+
+This change is only scoped to switching to OCI as the transport for Fedora CoreOS content.
+Derivation support is still a work in progress, see the tracking issue for more details : https://github.com/coreos/fedora-coreos-tracker/issues/1726
+
 == cgroups v1 support disabled
 
 In systemd v256, cgroups v1 support was disabled.

modules/ROOT/pages/update-streams.adoc

@@ -55,9 +55,8 @@ sudo systemctl stop zincati.service
 # Perform the rebase to a different stream
 # Supported architectures: aarch64, x86_64
 # Available streams: "stable", "testing", and "next"
-ARCH="$(arch)"
 STREAM="testing"
-sudo rpm-ostree rebase "fedora/${ARCH}/coreos/${STREAM}"
+sudo rpm-ostree rebase "ostree-remote-registry:fedora://quay.io/fedora/fedora-coreos:${STREAM}"
 ----
 
 After inspecting the package difference the user can reboot. After boot the system will be loaded into the latest release on the new stream and will follow that stream for future updates.