12.04 LTS and future releases

12.04LTS and further LTS releases are planned to have upgrades both for the desktop and server version for 5 years after release date.

You can read in the Ubuntu LTS wiki page:

A new LTS version is released every 2 years. In previous releases, a Long Term Support (LTS) version had 3 years support on Ubuntu (Desktop) and 5 years on Ubuntu Server. Starting with Ubuntu 12.04 LTS, both versions will receive 5 years support. There is no extra fee for the LTS version; we make our very best work available to everyone on the same free terms. Upgrades to new versions of Ubuntu are and always will be free of charge.

Non LTS release will still be maintained and will receive updates for 18 moths after release date.

Think of apt-get upgrade as a safe upgrade while testing candidate releases. You should use it normally for (mostly) daily upgrade process to keep your packages up to date without* breaking your system. The upgrade option is used only to update the packages already installed on your machine to a newer version.

apt-get dist-upgrade will not only upgrade all of the currently installed packages on your system but also handle the dependency changes for new versions of packages. It intelligently** will remove obsolete packages from your system and install any further necessary. During beta use this option with caution. Normally this function needed for upgrading from one distribution release to another.

_{* Without breaking is a big statement, remember, this is alphas or betas we are talking about.
** Intelligently is definition but because all is beta extra caution should be taken.}

Precautions to take while upgrading a pre-release

• Upgrade only using the terminal, at least you will know exactly what is going to happen before pressing Y and letting apt do changes to your system.
• When running a apt-get dist-upgrade is needed (new kernel, major change on system packages, etc) make sure that you first run apt-get dist-upgrade -s to simulate and please read all the changes the upgrade will apply. If you see any weird changes (_I remember killing my system after a dist-upgrade asked me to remove some necessary system packages and the Ubuntu Software Center while 12.04 was in alpha 1) abort it and update you packages again in a few days.
• Do not use the update manager while updating a pre-release. Make sure that you can read everything that is going to happen to your system before pressing Y.
• Don't be too eager to update! Don't do it out of speed, this is a sensible process during a pre-release. Use caution and some patience when testing it.

When should you use a upgrade compared with a dist-upgrade?

Use a dist-upgrade when you know the pre-release just went from alpha to beta or the next scheduled step or in the "Wont update these packages" list during a apt-get upgrade there are new kernels and other major packages, be extra cautious of the list of packages that will be removed. If there is anything there that you are not sure, or that you think it might break your system abort it after the simulation and get your self informed via the forums or on the IRC channels before applying the changes.

If you need further help during the update period or just a quick chat to point you in the right direction during an pre-release system read the informationins this post

• There's an issue with an Alpha/Beta Release of Ubuntu, what should I do?

Don't be hasty and be prepared all times, a backup or 2 are never too much to ask when testing.