From what I understand, Xcode has made it much easier for developers to transition to the iPhone 6 and 6 Plus sizes. I think it's called "auto layout" or something like that.
Autolayout helps for apps that use standard UI elements (scrolling lists, navigation bars, etc.). Where it's difficult is for apps that don't use those, mostly this means games. For that both new iPhones can involve some considerable work (although it really depends on the app). It's definitely not an impossible burden, but it's not necessarily trivial either. Autolayout is not a magic bullet that always works perfectly on whatever device you throw it on.