Today (14th April 2015) the Skype for Business Client update is getting rolled out as part of the Office Updates.
For existing Lync Online users the default UI will be Skype for Business Client from today!
Remember the client UI is controllable so if you want your users to retain the Lync Client look and feel access Lync Onlone via the Lync OnLine Management Shell and do the following
Disable Skype user interface (UI) for all users:
Grant-CsClientPolicy -PolicyName ClientPolicyDisableSkypeUI
For Lync On Premises Users
The default client will still be Lync Client.
Below is some great material and information available from Microsoft for users.