Microsoft has done a good job of telling people that change is coming to their Lync client. A quick recap of what is coming.
April 14th (Patch Tuesday) Microsoft is starting the process of pushing out the Skype for Business Client UI. This is going to hit “click to run” and those users who use Windows Update first. If you are a business organization you have more control because you are most likely using SCCM/WSUS to approve patches to your workstations and you can take a bit more time to deal with it.
Microsoft has several resource sites that you can use for more details on this. I suggest checking these out:
In these you will find the EnableSkypeUI option that allows you to change the UI of your client. However, there are a few things that will change regardless that the EnableSkypeUI doesn’t control.
Sounds
The installer will replace all of the Lync Sounds with the Skype Sounds. Users of both UI’s will experience this change. To me, this is the most notable change that your users will notice regardless. The sounds are VERY different then before. You should make sure your users are aware of this and your helpdesk is ready for the calls.
Meeting Links
If you are using Lync Conferencing your users are about to see a change. The Lync patch will update the Outlook Plugin and change the Lync Meeting to a Skype Meeting

And the actual join link will be changed as well

Initial Join
The default behavior of the Skype for Business Client is to launch into the Skype for Business UI. If you have selected via policy to use the Lync 2013 client you will see this:

This poses two problems. First, you need to inform all of your users they are going to see this annoying message. Second, it gives them the ability to choose Restart Later and lets them play in the new UI (which they are going to pick because end-users who see Restart will always pick later). The only way to suppress this is to push out the registry key before hand so the client never even asks.
This is the most notable visual change that users will see after the patch if they are using the Lync UI option.