IMPORTANT CU FOR SIDE BY SIDE MIGRATIONS TO SKYPE FOR BUSINESS
With the buzz of Skype for Business there’s also a new CU for Lync 2013 Server. This cu contains a number of fixes so would be recommended to get installed, also be good to have installed if your preparing your servers for a Skype for business in place upgrade.
KB Url
https://support.microsoft.com/en-us/kb/2809243
May 2015 CU Download URL
https://www.microsoft.com/en-us/download/details.aspx?id=36820
PLEASE NOTE – For Skype for Business Side by Side Migrations the May CU is required !!! There has been an issue found where when moving conference directories from Lync Server 2013 to Skype for Business Server 2015 conferencing data is LOST !!!!
Martin Boam's Microsoft Blog 