After you apply update 2919355 on a Windows 8.1 Pro-based computer that is not domain-joined, you can customize the Start screen. By default, only domain-joined computers in Windows 8.1 Pro can customize the Start screen.
This issue occurs because update 2919355 applies a Start screen customization policy incorrectly to the Pro edition of Windows 8.1.
To resolve this issue, install the November 2014 update rollup for Windows RT 8.1, Windows 8.1, or Windows Server 2012 R2.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
See How to customize the Start screen that applies to Windows 8.1, Windows Server 2012, and Windows Server 2012 R2.
See the terminology that Microsoft uses to describe software updates.