Well, depends what you mean by warmed over. Not a single piece of sheetmetal or interior panels are carried over, but as you can tell, the same basic structure and chassis design remain the same, albeit further tuned for better driving characteristics.
The interior is much improved, and loses all of the hard plastic of the current model in favor of a squishy dash.
Engines should be interesting. Like it's been said, the Mustang is all-new for MY2013, but new engines will be rolling out before then.
For MY2010, the V6 is replaced by a much hotter (and LIGHTER) 3.5L engine, while the V8 gets Ford Racing tune standard for a HP boost.
MY2011 gets the 5.0L V8 at long last, as well as the 3.5L turbo (in full-on 425hp tune).