Check out the history of body styling timeframe: 94-98, 99-04, 05-09, and even 10-14 had updates within that 5 year timeframe. There is a very good chance that ford will change the styling at least a little bit after a 5 year run. Same basically chassis, but could be a big body style change.
Yup, a mid cycle refresh, like I would expect in 2018, is pretty cheap compared to a redesign. Mid cycle refreshes typically have design/technology updates to keep the platform looking current without requiring the company to redo crash testing or EPA testing. Although they could decide to update the drivetrain enough for EPA testing, it wouldn't require crash safety testing. I would say there is a decent chance that we will see a 10 speed auto in 2018 in the mustang. I wouldn't bet money on it, but I would think after years of expensive development on the 10 speed, they would want to start getting it into cars ASAP to start recovering costs.