The Windows ADK 8.1 update (for Windows 8.1 Update) is available for download:
Windows ADK 8.1 update (direct download only: http://www.microsoft.com/en-us/download/details.aspx?id=39982
You still run adksetup.exe to install or download the updated ADK, but you do see that the new ADK is slightly bigger than the previous kit. The Patches folder content also have a higher version number. The October 18, 2013 release of Windows 8.1 ADK had a folder named 8.100.26020, but the April 2, 2014 release of Windows 8.1 ADK have 8.100.26629.
New features in ADK 8.1 are the WIMBoot option, updates to dism, updates to WinRE and a new WinPE version (5.1). There are also fixes for USMT.
DISM: Does not support Windows Vista or Windows Server 2008 images.
More info about the changes here: http://msdn.microsoft.com/en-us/library/windows/hardware/dn247001.aspx
Info on updating WinPE 5.0 to WinPE 5.1: http://technet.microsoft.com/en-us/library/dn613859.aspx