How do I revert to Kubuntu version 18.04 LTS from version 21.04 without losing anything?

18.04 was the last version I used that I liked. I used version 20.04 and currently use 21.04.

Ubuntu's apt and dpkg tools are designed to upgrade to later/newer packages, so the automatic tools will upgrade, but not downgrade.

You use data restoration tools (ie. restore your backups) when you want to go backwards.

You can re-install though :)

If you install using "Something else" (Manual I believe with the Qt skin over ubiquity that Kubuntu uses) and

• select your existing partitions but ensure format is unchecked

This will install with this procedure

• note your installed (additional) packages
• erase system directories
• install new system (whatever release you're using to install; which can be same release, earlier, or later)
• attempt to add back your added packages (if available in new release)
• ask you to reboot.

It won't touch any user files (unless you format!). As system directories are erased; many server apps can lose configs where they store conf files in system directories - but this isn't the case with desktop programs that Kubuntu provides.

You'll still need to check each of your used programs can cope with a backwards change; ie. I've had issues before doing this where my chosen email MUA (mail user agent) didn't know how to deal with later features I'd started using, thus emails where I'd use later features were ignored (didn't exist to earlier version). I've also had issues with RSS readers; so you'll need to check whatever programs you consider the data as valuable.

Easiest though is restoration of a backup (as is often the case)

Downgrading an existing Ubuntu system is practically not feasible. Thus, you need to reinstall an older version replacing the newer version, and restore settings, additional applications and your own user data.

The single most important thing is to have a good backup of your own data. You have this backup anytime. Not only when you want to downgrade. Once lost, personal data are lost forever. In contrast, an operating system or applications can be downloaded and reinstalled anytime.

Thus, reinstall your older version wiping the current version. Then place your data back, configure your system again and install the additional applications you need.

You cannot downgrade. This will break your system and land you in dependency hell if you attempt it. You can probably fix the problems this creates, but it's really not worth the effort. You can install 18.04 cleanly if you are really adamant that you cannot make do with a newer release.

If you want to go as far back as 18.04, you should probably stick with LTS releases instead of standard releases.

But first you should determine what it is exactly that you liked in 18.04 that you did not like in 20.04 or 21.04. It's more likely that you can improve your experience on in the newer release by making easy adjustments.

Consider that LTS releases are supported for 5 years, so you cannot just keep 18.04 on your system until the end of time. You will eventually need to upgrade to a newer release, so you're really better off articulating what you need to do in 20.04/21.04 that has made you believe you need to downgrade to achieve.