What I understood from the available documentation and my testing is that
All existing Apps on iPhone 6 and 6 Plus run in compatibility mode, which means th
You don´t have to use the launch screen file to make your App iPhone 6 / 6+ resolution compatible. Instead, you can select the LaunchImage asset as your Launch Images Source.
It can be found at "App Icons and Launch Images" under your Targets:
If there is no LaunchImage asset just go to your Images.xcassets, make a secondary click (right click) and select "New Launch Image":

The result is something like that:
Now just drag and drop your images for the specific resolutions you want to support and set the created LaunchImage asset as your source.
Hope it helps
Cheers