Hello,
I tried changing
add_image_size( 'twentyfourteen-full-width', 1038, 576, true );
as bshope did
to
add_image_size( 'twentyfourteen-full-width', 1038, 300, true );
and it worked for me with images uploaded after the change. They were cropped to the new dimension correctly.
Images from before the change got scaled down (width and height) and needed another upload. Or maybe a plugin helps to change them correctly.