This needs to be said: if you own cases for the iPhone 7 or 7 Plus, you can use them
when if you upgrade to the iPhone 8 or 8 Plus. Their designs are similar, ports and camera placements are unchanged, and really, it’s amazing how Apple retained the dimensions to be virtually identical.
A quick check at Apple’s official product pages for the iPhone 7 and iPhone 8 reveals that despite the new glass back and internal hardware, there is – at most – only a 0.2mm difference in dimensions between the two models (and their respective Plus variants). In the case of the iPhone 7 and 8’s length, it’s just 0.1mm.
In fact, the Apple online store has been updated to reflect the new iPhone. All iPhone 7 cases – as well as the new colours for existing cases – will mention either “iPhone 7/iPhone 8” or “iPhone 7 Plus/iPhone 8 Plus” in its product name.
Besides that, the same applies to the Apple Watch as well. The proprietary straps (in both 38mm and 42mm sizes) are interchangeable across the series, which means the strap from Series 1 can be used on Series 3.
With Apple usually retaining the same design iPhone for two generations, it isn’t really a surprise to see Apple doing the same for the iPhone 7 and iPhone 8; the 8 is essentially a 7S model anyway. On top of that, there’s the massive, massive market for iPhone accessories as well – changing designs every year means consumers would have to pay more to replace their obsolete cases and covers.