It is possible to move back users that have been moved from the on-premises Lync deployment to the cloud, and it is also possible to move on-premises users that have been defined and enabled directly in Office 365. In the latter scenario, it is important to create the user also in the on-premises domain (Directory Service).
As we have seen in other scenarios, it is required to have Active Directory synchronization and Federation Services correctly configured. Refer to the Authenticating with online services using DirSync recipe in Chapter 2, Lync 2013 Authentication.
It is required to federate Lync Server 2013 with Lync Online, as we have done in point 5 of the How to do it… section in the Planning and configuring a hybrid deployment recipe.