I had heard that Ford will offer "special" models for the immediate future, followed by a mild makeover and finally a fairly significant restyle around the time the Camaro arrives, with the only body panel remaining "as is" being the roof.
I would think Ford would do something with the Mustang seeing as the current body style is well into its life cycle. Unless there is a significant change, the current body style will look "dated" when the Camaro arrives.