An in-place upgrade is the most complicated form of Windows Server 2012 R2 installation. It is also the lengthiest and the most likely to cause problems during its execution. Whenever possible, Microsoft  recommends that administrators perform a clean installation or migrate required roles, applications, and settings instead.
Although in-place upgrades often proceed smoothly, the complexity of the upgrade process and the large number of variables involved means that there are many things that can go wrong. To minimize the risks involved, it is important for you to take the upgrade process seriously, prepare the system beforehand, and have the ability to troubleshoot any problems that might arise. The following sections discuss these subjects in greater detail.

Upgrade paths
Upgrade paths for Windows Server 2012 R2 are limited. In fact, it’s easier to specify when you can perform an upgrade than when you can’t. If you have a 64-bit computer running Windows Server 2008 or Windows Server 2008 R2, you can upgrade it to Windows Server 2012 R2 as long as you use an appropriate operating system edition.
Windows Server 2012 R2 does not support the following:
– Upgrades from Windows Server versions prior to Windows Server 2008
– Upgrades from pre-RTM editions of Windows Server 2012 R2
– Upgrades from Windows workstation operating systems
– Cross-platform upgrades, such as 32-bit Windows Server 2008 to 64-bit Windows Server 2012 R2
– Upgrades from any Itanium edition
– Cross-language upgrades, such as from Windows Server 2008, U.S. English to Windows Server 2012 R2, French
In any of these cases, the Windows Setup program will not permit the upgrade to proceed.

Preparing to upgrade
Before you begin an in-place upgrade to Windows Server 2012 R2, you should perform a
number of preliminary procedures to ensure that the process goes smoothly and that the
server data is protected.
Consider the following before you perform any upgrade to Windows Server 2012 R2:

Check hardware compatibility Make sure that the server meets the minimum hardware requirements for Windows Server 2012 R2.
Check disk space Make sure that there is sufficient free disk space on the partition where the old operating system is installed. During the upgrade procedure, sufficient disk space is needed to simultaneously hold both operating systems. After the upgrade is complete, you can remove the old files, freeing up some additional space.

Confirm that software is signed All kernel-mode software on the server, including device drivers, must be digitally signed or the software will not load. This can result in an aborted upgrade process, hardware failures after the upgrade is completed, or failure of the system to start after the upgrade. If you cannot locate a software update for the application or driver that is signed, then you should uninstall the application or driver before you proceed with the installation.

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Note: DISABLING THE DRIVER SIGNATURE
If an unsigned driver prevents the computer from starting, you can disable the driver signature requirement by pressing F8 during the startup, selecting Advanced Boot Options, and then selecting Disable Driver Signature Enforcement.

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Save mass storage drivers on removable media If a manufacturer has supplied a separate driver for a device in your server, save the driver to a CD, a DVD, or a USB flash drive in either the media root directory or the /amd64 folder. To provide the driver during Setup, click Load Driver or press F6 on the disk selection page. You can browse to locate the driver or you can have Setup search the media.

Check application compatibility The Setup program displays a Compatibility Report page that can notify you of possible application compatibility problems. You can sometimes solve these problems by updating or upgrading the applications.
Create an inventory of the software products installed on the server and check themanufacturers’ websites for updates, availability of upgrades, and announcements regarding support for Windows Server 2012 R2. In an enterprise environment, you should test all applications for Windows Server 2012 R2 compatibility, no matter what the manufacturer says, before you perform any operating system upgrades.

Ensure computer functionality Make sure that Windows Server 2008 or Windows Server 2008 R2 is running properly on the computer before you begin the upgrade process. You must start an in-place upgrade from within the existing operating system, so you cannot count on Windows Server 2012 R2 to correct any problems that prevent the computer from starting or running the Setup program.

Perform a full backup Before you perform any upgrade procedure, you should back up the entire system or, at the very least, the essential data files. Your backup should include all data and configuration information that is necessary for your target computer to function. When you perform the backup, be sure to include the boot and system partitions and the system state data. Removable hard drives make this a simple
process, even if there is not a suitable backup device in the computer.

Disable virus protection software Virus protection software can make installations much slower by scanning every file that is copied locally to your computer. If installed, you should disable this software before performing the upgrade.

Disconnect the UPS device If you have an uninterruptible power supply (UPS) connected to your target computer, disconnect the data cable before performing the upgrade. Setup automatically attempts to detect connected devices; UPS equipment can cause issues with this process.

Purchase the correct Windows Server 2012 R2 edition Be sure to purchase the appropriate Windows Server 2012 R2 edition for the upgrade and have the installation disk and product key handy.

During the upgrade process, when the system restarts, the boot menu provides an option to roll back to the previous operating system version. However, once the upgrade is complete, this option is no longer available and it is not possible to uninstall Windows Server 2012 R2 and revert to the old operating system version.

This article is a part of 70-410 Installing and Configuring Windows Server 2012 Prep course, more articles in this course are :

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70-410 Installing and Configuring Windows Server 2012 Prep course includes following practice tests:

Exam 70-410 Installing and Configuring Windows Server 2012 Practice Test 1

Instructions for Exam 70-410 Installing and Configuring Windows Server 2012 Practice Test 1 This page shows the instructions for Exam ...
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Exam 70-410 Installing and Configuring Windows Server 2012 Practice Test 2

Instructions for Exam 70-410 Installing and Configuring Windows Server 2012 Practice Test 2 This page shows the instructions for Exam ...
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Exam 70-410 Installing and Configuring Windows Server 2012 Practice Test 3

Instructions for Exam 70-410 Installing and Configuring Windows Server 2012 Practice Test 3 This page shows the instructions for Exam ...
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Exam 70-410 Installing and Configuring Windows Server 2012 Practice Test 4

Instructions for Exam 70-410 Installing and Configuring Windows Server 2012 Practice Test 4 This page shows the instructions for Exam ...
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Exam 70-410 Installing and Configuring Windows Server 2012 Practice Test 5

Instructions for Exam 70-410 Installing and Configuring Windows Server 2012 Practice Test 5 This page shows the instructions for Exam ...
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