Linux users have long debated whether it is better to perform an in-box upgrade or completely wipe a system clean and reinstall whenever there is a new major version release.
For desktop Linux users, the answer is matter of convenience. For server administrators, it can make or break a career or business.
Many Linux distributions have become very good at performing in-box upgrades without much interference with the running applications or many problems. Operating systems like CentOS and Debian have package managers (yum and apt) that can upgrade packages rather easily. The only problem with this is that, if something goes wrong, there is no easy way to roll back to the previous version.
The other option, completely starting over, is the best way to ensure you have all of the right packages, drivers, and hardware compatibility. But this method is much more intrusive, requires you to backup everything on your server to another location, and worst of all, causes tremendous down time.
Having said that, someone using a remote server may not even have the option of reinstalling the operating system himself, rather the web host may re-image the machine to the latest version. Still, it would require the administrator to reinstall any previously used third-party software, as well as all of the websites. The chance for error becomes great whenever you add more tasks that must be redone and reconfigured.
In reality, the best solution is to keep the OS you have, unless you have a true legitimate reason to upgrade. Many Linux distributions made for servers have long release cycles, continual security updates, and are supported for three, five, and sometimes even seven years. In other words, you may very well upgrade your hardware before you need to upgrade the OS. If you absolutely must have the latest, for whatever reason, an in-box upgrade is probably the easier solution.