Windows 8.1 Update 1 is now available in Microsoft's Volume License Service Center. The updated ISO will replace the one originally made available to VLSC in October 2013.
Windows 8.1 Update 1 is a cumulative update, so the new ISO should be used to replace the one stored for image creation now. Instead of just updating your current images with the separate files delivering on April 8, Microsoft is recommending a full swap. Just use the new ISO.