SQL Server Edition Upgrade

SQL Server edition upgrade is a process in which an SQL Server instance will be upgraded from one edition to another.
To find the supported paths for Version and Edition Upgrades, follow the below link to make sure to choose the right upgrade path.
http://msdn.microsoft.com/en-IN/library/ms143393.aspx

The steps in this post, shows the procedure to upgrade SQL Server 2008 R2 Standard Edition to Enterprise Edition.

Before the Upgrade:

Edition Upgrade Process:

1. Insert the Installation media and Run "setup.exe"
2. Go to "Maintenance" page and choose "Edition Upgrade"
3. This will now check and verify few rules which help in the smooth setup process. Click "Ok" in this screen
    
4. On clicking ok, the setup will now verify few more rules. Click "Next" in this screen
   
5. Enter the Product Key and click "Next"
   
6. Read the License Terms and accept it and click "Next"
   
7. Select the Instance that needs to be upgraded and Click "Next"
   
8. On clicking next in previous step, the setup verifies few rules related to upgrade, based on the result, fix any issues reported or click "Next"
   
9. In the summary screen, verify the actions, if any changes required click "Back" else click "Upgrade"
   
10. Once the Upgrade is completed, Click "Close"
    

After Upgrade:

A failure was detected for a previous installation, patch, or repair during configuration for features...

You receive the following error when you try to apply an update or service pack for a SQL Server Instance.

TITLE: SQL Server Setup failure. ------------------------------ SQL Server Setup has encountered the following error: A failure was detected for a previous installation, patch, or repair during configuration for features [SQL_Engine_Core_Shared,SQL_PowerShell_Engine_CNS,CommonFiles,CommonFiles_Loc,]. In order to apply this patch package (KB2528583), you must resolve any issues with the previous operation that failed. View the summary.txt log to determine why the previous operation failed. Error code 0x84B20001. ------------------------------ BUTTONS: OK ------------------------------

This happens when you try to install an update or service pack or do an edition upgrade and if that action does not complete successfully.
In my case, I recently tried to do an upgrade of SQL Server 2008 R2 Evaluation version to SQL Server 2012 Evaluation edition and since this upgrade path was not supported, the installation gave an exception which forced me to quit the installer.
After this I tried to install SP1 for SQL Server 2008 R2 and I got this error.

To Fix this issue, you have to "Repair" the existing SQL Server Instance. This option is available in "Add Or Remove Programs" feature of Control Panel.

Once the Repair is completed, you can restart the installation of Service Pack.