Looking at what Nissan did with the '15 Nismo. Tells me that it will be another 2 years or so before they release an all new Z.
Why would they spend the money on a few up-date peices if they wasn't planning on keeping it the same for awhile longer. It will take about 2 years to get their money back on the design change on the new stuff. Also, I think they are testing the waters too with the design changes.