Summary
After you install this update, you will no longer be able to add Microsoft Skype for Business Server 2019 server roles to an environment that contains both Lync Server 2013 and Skype for Business Server 2015 server roles. When you try to deploy Skype for Business Server 2019 in tri-existence mode, you will be notified that it's an unsupported configuration.
How to get this update
To get this update, install the December 2019 cumulative update 7.0.2046.151 for Skype for Business Server 2019, Administrative Tools.
