Because 14.04 is EOL (End of Life) there are special considerations:
Upgrade or fresh install?
The advantage of upgrading is that you get to keep your current
configuration as is. This is often desirable on production
environments which you want to keep as stable as possible. At the same
time, configuration applicable for older versions might not always
work with newer versions.
If you format partitions when installing a newer version, you have the
option to select a new file system, which might have benefits and
required features compared to the old one. The new installation also
gives you a clean platform to start building your system from; no old
configuration files and potential upgrade-related bugs.
Reinstalling is usually easier and faster, especially if you would
have to upgrade through several releases. It might also help you save
some bandwidth, if that is a concern.
That said if you were to do a clean install you might want to do it on a new partition and keep your old partition intact. This is helpful for migrating settings set many years ago on the old partition but unknown for the new partition. For example you may have custom scripts in
/usr/local/bin or custom settings in
/etc which you setup many years ago and then forgot about until the new installation isn't working properly.
If you want to upgrade you should consider cloning your old partition to the new partition and run the upgrade on the clone. Then if the upgrade fails, you can reclone, fix the error condition and run the upgrade again.
Please note an upgrade can take 40 minutes (in my case) to a few hours (reported by other users) depending on your system.
A few times a day, a few times a week, month and year I will change different system settings. Over the course of five years I forget all that I've changed. The advantage of upgrading is none get lost. The disadvantage of reinstalling is all get lost.