In those two big chip transitions, Apple prevailed by using nearly transparent emulation and other clever tricks, like "fat binaries" that bundled two versions of the same program in a single executable package, one for each processor. True, each transition took software developers on a wild ride, but most users found the switch tolerable and often seamless.
Still, the difficulty of swapping CPU architectures should not be underestimated. Although Apple can undoubtedly do it again, switching the Mac to ARM may not gain it the same advantages as previous switches. Remember that in the two previous switches, the old architecture was falling way behind the performance curve, but that's not so with the x86 today. Indeed, Intel is still leading the curve.
Why Apple is unlikely to switch iOS to Intel processors Rumors that Apple will port iOS to the x86 are abetted by the steady improvement of Intel's Atom-based chips for smartphones and tablets. Keep in mind that Intel and ARM started from opposite ends of the spectrum: Intel from high performance, ARM from low power. Now each company is moving toward the other's position and have nearly met in the middle.
Intel's Atom processor, introduced in 2008, was the company's first attempt to drastically reduce x86 power consumption. Initial Atom chips were great for netbooks but still way too hot for smaller devices like smartphones. In 2010, Intel's "Lincroft" processor (officially, the Atom Z600 series) moved graphics, video coding, DRAM control, display control, and I/O interfaces onto the same chip as the Atom CPU core. For Intel, this was a big step in chip integration and power efficiency, but "Lincroft" still ran relatively hot and required a companion chip to match the functions of competing single-chip processors.
In 2012, Intel introduced "Medfield" (officially, the Atom Z2460). This chip finally integrated the critical smartphone application functions on a single chip and cut power consumption to competitive levels. Even so, it's not quite good enough to convince most smartphone vendors to rewrite their ARM software for the x86. In fact, Intel has had to take on the work of porting Android to Atom; despite that effort, very few smartphones or tablets use it -- even though most Android apps are written in Java, which bridges chip platforms more easily than natively compiled code.