Upgrading Debian and Ubuntu to the next major release without losing a rollback path

Goal

Move an Ubuntu Server 22.04 host to 24.04, or a Debian 12 (bookworm) host to 13 (trixie), with a working way back if the upgrade fails partway.

Prerequisites

Full backup or snapshot — Debian's release notes call this "strongly recommended" before upgrading, warning that a hardware failure mid-upgrade can leave a system "severely" broken; a console (serial, KVM or provider web console) independent of SSH, in case networking changes mid-upgrade; enough free disk space for downloaded packages.

Steps

Ubuntu

1. Ensure all current updates are installed: apt-get update && apt-get -y upgrade.
2. Run do-release-upgrade. Ubuntu's documentation recommends this command specifically for Server edition and cloud images because it "can handle system configuration changes that are sometimes needed between releases."
3. Between two LTS releases, do-release-upgrade only offers the next LTS once its first point release ships; to test a development release instead use -d, which Ubuntu's docs say is "not recommended for production environments."
4. Over SSH the tool runs itself inside screen so the upgrade survives a dropped connection (reattach with screen -r). Keep the original SSH session open and, from the separate console, watch for the temporary SSH service the tool starts on an alternate port in case the primary session drops.

Debian

1. Start a tmux or screen session first, so a dropped SSH connection does not kill apt mid-upgrade. Update /etc/apt/sources.list (or the .sources files) to replace the old release codename with the new one, then apt-get update.
2. Do a minimal pass first: apt-get upgrade --without-new-pkgs, which "upgrad[es] those packages which can be upgraded without requiring any other packages to be removed or installed" — a good point to catch disk-space or held-package problems early.
3. Complete the upgrade: apt-get full-upgrade (or apt full-upgrade), which allows removals and new installs.
4. Reboot once the process finishes cleanly.

Expected result

lsb_release -a (Ubuntu) or cat /etc/debian_version (Debian) reports the new release; apt list --upgradable returns nothing outstanding.

Limits and test basis

Both procedures need a reboot to load the new kernel and finish systemd unit transitions. If the upgrade aborts, restoring the backup taken in the prerequisites is the documented fallback; neither vendor claims a guaranteed automatic rollback path.