Updated models with AMD graphics options expected in early 2017.
iPhone 5S Changes Highlighted in New Rear Shell Comparison Photos
One example shows an overlay of the iPhone 5S bare logic board, which has leaked several times in recent months. The part contains a number of repositioned screw holes compared to the iPhone 5 logic board, and overlay photos shows how these holes line up exactly with mounting points on the rear shell.
Also of interest is the home button region, as Apple has been reported to be incorporating a fingerprint sensor into the part, perhaps moving to a sapphire-covered convex design to help accommodate the sensor.
Nowhereelse.fr's comparison photos show how the metal spacer shield for the home button has changed from the iPhone 5 to the iPhone 5S, with the previous nearly symmetric design moving to an asymmetric design with altered screw hole locations and an angled edge along one side.
The exact reason for the design changes on the spacer is unclear given a lack of other parts, but it seems reasonable to suspect that they may be related to accommodating the fingerprint sensor.
Apple's September 10 media event is expected to include the introduction of the iPhone 5S and perhaps the "iPhone 5C", a lower-cost model built around a plastic shell. Apple is also preparing to introduce new iPads in the coming months, although those are not expected to debut at this event.