If you receive a message like this:
You have the newer version of Lync called Skype for Business. However, your admin would like you to use Lync or it’s the only version your server supports. Please restart now to use Lync.
If you select “Restart later” you can use the Skype UI and everything seems to work, but on restarting Lync you are back to the old Lync UI.
The proper way to change this is on the Lync backend.
But if you don’t have access to it a temporary workaround is to set EnableSkypeUI to 00 00 00 01 under HKEY_CURRENT_USER\Software\Microsoft\Office\Lync
However this will get reset back to 0 each time you launch Skype for Business.
To fix this, set it to 1, then deny your user account “Set Value” permission. Do this by right clicking Lync folder in registry and select Permissions –> Advanced –> Add, then select Advanced permissions and Deny Set Value.
You will still get the prompt to restart Skype into Lync mode, just that it will always remain in Skype UI.