Ask most IT professionals if it is better to do an in-place upgrade to Windows 8 or to perform a clean installation, and most will tell you that you should always go with a clean install. While I generally agree with this philosophy, this would be an extremely short article if there weren't a bit more to the story.
The main reason why experienced IT professionals tend to recommend performing a clean installation of a new operating system is because clean installations tend to be more reliable. Anytime that you perform an in-place installation, there are certain things that carry over from the old operating system to the new operating system. This "baggage" obviously includes applications, but it can include other things as well, such as hardware drivers or malware, both of which can be problematic. When you perform a clean installation of a new operating system you can rest assured that the new operating system is not trying to use outdated drivers that were intended for the old operating system and that there aren't any malware infections present.
Another advantage behind performing a clean installation is that you can ensure a consistent end-user experience. Typically all but the smallest organizations perform image-based deployments of desktop operating systems. In other words, the administrator installs Windows onto a physical or virtual machine, adds drivers and applications, and then packages the operating system and applications for distribution. This method is much less time consuming than upgrading each desktop individually. More importantly, however, the administrator is able to meticulously test the deployment image prior to rolling the image out to the end users. This helps to ensure that the end user's desktop will be stable and reliable after the new operating system has been installed.
After reading the previous three paragraphs, it probably seems as though it is better to perform a clean installation of a new operating system than to try to perform an in-place upgrade. In most cases, a clean installation really is the best method of deploying Windows 8. However, there are definitely some circumstances that warrant an in-place upgrade.
Sometimes applications, especially legacy systems, can dictate the need for an in upgrade. One of the steps that I always take when preparing to transition to a new operating system is to run a software inventory of the machines that are to be upgraded. When I inventoried the desktops in question, there was one particular application that I didn't recognize.
When I spoke to the guy in charge about the application, he informed me that the application was an accounting package that had been custom written for the company. I asked him if he had a copy of the installation media so that I could test the application prior to performing the upgrade. This is where things got interesting.
Apparently the application had been developed quite some time ago. The developer had provided the application on a 3.5 inch floppy disk that had since become demagnetized. Worse still, the firm that had developed the application had later gone out of business, so there was no way to get a new copy of the application.
Given the circumstances, I didn't want to move forward with the operating system upgrade, but the company was insistent even after I had explained the risks. Needless to say, I performed a full system backup of each computer prior to the upgrade just in case anything went wrong.
I was also forced to do an upgrade since there was no way to reinstall the application. In order to minimize the risks, I replaced each computer's hard drive with a brand new drive and restored the backups to the new drives. I labeled the old hard drives and put them somewhere for safe keeping. That way, if anything went wrong with the upgrade, I could simply reinstall the original, unaltered hard drives and it would be as if nothing had ever happened. Thankfully, the operating system upgrade went off without a hitch and the application worked perfectly with the new operating system.
Another factor that can sometimes justify an in-place upgrade is if PCs are running especially complex configurations. When I was with the military, there was one particular unit that used a lot of proprietary peripherals and that used a rather complex network configuration (sorry, I am not allowed to discuss the details). When it came time for an operating system upgrade, the decision was made that it would be less risky and less expensive to do an in-place upgrade than to do a clean installation and a lot of specialized configuration work.
When it comes to deciding whether to do a clean installation of Windows 8 or an upgrade, you have to do what makes the most sense for your own unique circumstances. Clean installations are generally recommended, but they are not always practical. Regardless of which method you decide to use however, it is important to test your implementation plan prior to moving forward with the deployment.