Here is a setup of the HD plugin in cooperation with the new YouTube API that was introduced in JW Player 5.6. If a YouTube video is loaded, the plugin automatically detects if an HD version of the video is available. If so, the toggle is displayed:
Note that YouTube offers up to 5 version of a video (240p, 360p, 480p, hd720 and hd1080). The HD plugin offers a toggle if at least the hd720 is available. This toggle is always performed between the 360p and hd720 versions.
Note: You must be using JW Player 5.6 for this feature to work.