Windows Azure Cloud OS is available in following two Windows Server 2008 Versions:
Older Windows Azure Cloud OS versions 1.0 - 1.11 are available with Windows Server 2008 SP2. (OLDER). Newer Windows Azure Cloud OS version 2.0 - 2.2 are Available with Windows Server 2008 R2 (NEWER). If you are using Windows Azure SDK 1.3 or( newer 1.4) you will get newer version of OS.
If you decided to change Cloud OS version by any reason you can follow these steps:
1. Changing Cloud OS version to Windows Server 2008 R2
2. Changing Cloud OS version to Windows Server 2008 SP2