This is the process: https://technet.microsoft.com/en-us/library/jj204969.aspx?f=255&MSPPError=-2147217396
If they were created in Skype for Business Server or Lync Server before being moved online, they will show in the on-premises SFB/Lync as being homed online.
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SWC Unified Communications This forum post is based upon my personal experience and does not necessarily reflect the opinion or view of Microsoft, SWC, their employees, or other MVPs.