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Question – Can I do an In-Place upgrades to Windows 8 or Windows Server 2012 ?
Answer – Yes you can, but the some old rules still apply – In-place upgrades from 32-bit edition of Windows Server 2008, upgrades from one language to another, or from one build type to another (chk to fre) are not supported.
Now to answer the other question – “Should I upgrade my OS ?”
Answer – It depends, I know what a great answer. However my answer has recently changed, in the past it was ALWAYS NO. However, I realize that there will be times when doing an in-place upgrade may be the easiest way, or only way you can get a machine to the new OS. I did some checking around some of my resources and found what I consider the definitive answer on these questions. Coming straight from the horses mouth as the saying goes: The Windows Server Core Team.
The team has created 2 different blog post series one on Upgrading to Windows Server 2012 and another one on Upgrading to Windows 8:
Upgrading to Windows Server 2012
Upgrading to Windows 8
I am still a huge fan of doing a clean install on any O/S when possible, but hopefully this information will help you make the decision – In-Place Upgrade or Clean Install.
Windows Core Team Blog – http://blogs.technet.com/b/askcore/