There are several limitations to be aware of before upgrading to SQL
Server 2012. The most important limitations are:
• Windows Server 2003 isn’t a supported OS. If your instance of
SQL Server is running on a Windows Server 2003 system, you
must upgrade the OS before upgrading to SQL Server 2012.
• A cross-platform upgrade isn’t supported. You can’t upgrade a
32-bit version of SQL Server to a 64-bit version.
• You can’t add new features as a part of the upgrade process. However,
you can add new features after the upgrade.
• The Windows Installer service must be running.
• Microsoft .NET Framework 3.5 SP1 and .NET Framework 4.0 are
• Windows PowerShell 2.0 or later is required if you’re upgrading
the database engine.
• IIS 7.0 or later is required for the Microsoft Management Console
(MMC), SQL Server Data Tools, Report Designer in SQL Server
Reporting Services (SSRS), and HTML help.
For more information about the prerequisites for installing SQL Server
2012, you can refer to “Hardware and Software Requirements for
Installing SQL Server 2012.”
Source : SQL Server Pro : May 2013 edition