Thanks everyone. I finally got on to Emirates via a Live Chat and the agent confirmed what you have all said. Which is totally different to what "James" said when I phoned the local Australian office.

My review date is 31/12/18, and my digital card expires 28/2/19 so I presume I keep the benefits til then. I am unsure though if I will return to Silver once we are back from our return flight. It depends if they are talking tier points earned in a 12 or 13 month period. If it is 12, then I won't have enough, but if it is 13 (from 1/1/18 to 31/1/19) then I do.