Cropping the image correctly won't distort it. How much you can crop is best determined by checking the result; maybe by a third. That is influenced by the source (native resolution of the camera). In theory you could crop by 720/1440, or 576/1080.
it's a great way to do what you want! I've paned/cropped an HD for SD & it looks great! like said, you can zoom in to the SD res you're working on until it starts to loose resolution.