Lots of people (including Office365 users) should have been moved to Lync 2013 by now, though the impact may not be all that obvious, since some of us have been using Lync 2013 client for a while, even if the back-end wasn’t running the latest and greatest.
Some of the changes will only be apparent when you join a Lync 2013 meeting – for example, when an application is being shared, one view will show you a line of photos of active participants in the meeting, and when each is talking, a green bar is shown under their mugshot, so you know who’s making all that noise, heavy breathing into their microphone, sniffling etc. Subtle.
[NB: in the photo below, the only highlighted person was actually talking at the time, and if fact, was the only one showing real video rather than a static photo]
There are a bunch of incremental improvements which the server upgrade provides, and some which subsequent updates to the client light up – one of which is the “Meetings” section which appears under the your status section at the top of the main Lync window (and somewhat confusingly, is depicted like a pie chart). The meetings tab shows you the remainder of your diary for the day (refreshed every 10 mins), and helpfully highlights any Lync meetings in blue, so you can right-click to Join. No need to go back to Outlook, open the calendar appointment, click on the link etc.
Other tweaks include the ability to set whether the IM and participant list shows up when you join a meeting (both of which were previously hidden by default until you clicked around inside the meeting). And if you’re the meeting organiser, you could decide to stop IMs or video in a meeting/call altogether.
The behaviour of the Lync client when you join a meeting is set under the Options icon (towards the top right, or you can press the ALT key to show the menu bar and go in throught Tools / Options if you’re old skool). 
There’s also the new ability to choose which Lync client to use for joining meetings – handy if you also have the Lync Modern app installed and would rather use it, for example.
Oh, and don’t forget to install and configure the fab Lync 2013 for Windows Phone, too. As before, but with more panache and pizzaz, the Lync client allows you to join conference calls with a single click (no typing in participant numbers and the like), as well as adding some cool new functionality – like using Wifi for VoIP calls if you set it to.
