After you move a Lync user from an on-premise Microsoft Lync Server 2013 server to Microsoft Lync Online, the URL of the user's online meetings is not updated automatically. More specifically, when the user creates a new Lync Online meeting or opens an existing meeting in Microsoft Outlook, the URL of the online meeting incorrectly points to the on-premise Lync server.
This behavior is by design. The URL of online meetings is not updated until the user restarts Outlook.
To resolve this issue, the user must restart Outlook and then update the online meeting invitations for existing meetings when the user is prompted to do this. After the user updates the online meeting invitations for existing meetings, the updated URL is used when the user creates new meetings.