There is a bit of a misconception out there regarding 64-bit processors. This is from the linked BBC News article:

However, bearing in mind there will remain many iOS and Android handsets on the market that still rely on 32-bit chips, this may deter developers from taking advantage of the switch in the short-term.

“People who have the old 32-bit processors will not be able to run software that is built specifically for the 64-bit processors because the latter uses a different instruction set,” explained Prof Alan Woodward, from the University of Surrey’s computing department.

“However, if people write in 32-bit, it will run on many of the 64-bit processors because they still support the old instruction set.

“So, you can get the whole market by writing the app in 32-bits, but you can only get a very small part of the market if you write specifically for 64-bits.”

This is a bit misleading. Apple’s developer tools make it reasonably easy to create a build that supports multiple architectures, say a 64-bit iPhone 5S as well as a 32-bit iPhone 5. If the app is built properly, it will work on both architectures.

In other words, it just works.