The steps are taken from this page https://wiki.almalinux.org/elevate/ELevating-CentOS7-to-AlmaLinux-9.html

To upgrade to AlmaLinux 9, you will need to migrate to AlmaLinux 8 first.

sudo curl -o /etc/yum.repos.d/CentOS-Base.repo https://el7.repo.almalinux.org/centos/CentOS-Base.repo
sudo yum upgrade -y

Once yum finishes, reboot

sudo reboot

Now install elevate-release and leapp packages

sudo yum install -y http://repo.almalinux.org/elevate/elevate-release-latest-el$(rpm --eval %rhel).noarch.rpm
sudo yum install -y leapp-upgrade leapp-data-almalinux

Let’s run a pre upgrade check to see if there are any errors.

sudo leapp preupgrade

If everything checked out ok, proceed with the upgrade.

sudo leapp upgrade

Once finished, reboot.

sudo reboot

Once the system is booted, verify it upgraded.

cat /etc/*release

Problems

https://wiki.almalinux.org/elevate/ELevate-frequent-issues

LUKS

If you are using LUKS and encounter an error, check out the following link.

https://www.it-hure.de/2024/02/update-alma-rhel-with-leapp-and-luks/

You can disable the check with the following command.

rm -rf /usr/share/leapp-repository/repositories/system_upgrade/common/actors/inhibitwhenluks

More space needed on / filesystem

Try setting LEAPP_OVL_SIZE to 4096

export LEAPP_OVL_SIZE=4096

Then launch the upgrade with

sudo --preserve-env leapp upgrade

https://forums.almalinux.org/t/at-least-48mb-more-space-needed-on-the-filesystem-running-leapp-upgrade/3808/2

You can also try removing files to create more free space.

https://github.com/oamg/leapp/issues/778

Upgrading to 8.8 (Or 8.4)

For the same reason, we recommend upgrading your CentOS 7 machine to AlmaLinux OS version 8.8. To do so, you need to navigate to the /etc/leapp/files/ directory and edit the leapp_upgrade_repositories.repo to lower the AlmaLinux version in baseurl/mirror to 8.8.

The 8.8 repositories are archived. To upgrade to AlmaLinux 8.8, change ‘/etc/leapp/leapp_upgrade_repositories.repo” to the following.

[almalinux8-BaseOS]
name=AlmaLinux 8 - BaseOS
baseurl=https://vault.almalinux.org/8.8/BaseOS/$basearch/os/
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-AlmaLinux

[almalinux8-AppStream]
name=AlmaLinux 8 - AppStream
baseurl=https://vault.almalinux.org/8.8/AppStream/$basearch/os/
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-AlmaLinux

[almalinux8-PowerTools]
name=AlmaLinux 8 - PowerTools
baseurl=https://vault.almalinux.org/8.8/PowerTools/$basearch/os/
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-AlmaLinux

[almalinux8-HighAvailability]
name=AlmaLinux 8 - HighAvailability
baseurl=https://vault.almalinux.org/8.8/HighAvailability/$basearch/os/
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-AlmaLinux

[almalinux8-ResilientStorage]
name=AlmaLinux 8 - ResilientStorage
baseurl=https://vault.almalinux.org/8.8/ResilientStorage/$basearch/os/
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-AlmaLinux

[almalinux8-Extras]
name=AlmaLinux 8 - Extras
baseurl=https://vault.almalinux.org/8.8/extras/$basearch/os/
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-AlmaLinux

If you have issues, you may consider upgrading to 8.4 first. To do that, change 8.8 to 8.4, and comment out the ResilientStorage section.

GPG Key failing after upgrade

Try importing the AlmaLinux 8 GPG key

rpm --import https://repo.almalinux.org/almalinux/RPM-GPG-KEY-AlmaLinux

https://almalinux.org/blog/2023-12-20-almalinux-8-key-update