Apple isn't putting a gun to developers' heads, but iOS 6 apps need to be upgraded for iOS 7
For those in the process of designing apps for the iPhone, Apple suggests writing for iOS 7 first, and then make adjustments for iOS 6 if needed. And while Apple hasn't posted a specific deadline or due date for developers to submit changes, iOS 7 is going to be launched this fall.
Apps will need to be upgraded to reflect the new color scheme. Menus, buttons, icons, and navigation keys will need to be revamped. This is not the time to be lazy because iPhone users are not going to be inclined to install any app that doesn't mesh well with the new UI. Still, Apple is not forcing developers to update their titles which could lead to a bifurcated App Store.
source: Thisisentropy via GIGaom