An update for the Lync 2013 app for Windows Phone was just released, taking the app to version 5.9.1371.0.
I noticed no major changes (could be quicker but I believe this is only wishful thinking…), however the app description states clearly that Lync will soon become Skype for Business:
When the app is started there’s a one-time screen confirming a change is coming:
Download the update from Windows Store.