The High Quality Streaming option is not displayed correctly upon changing from a premium to free/unlimited account in the Desktop client.
Log in as a Premium account, ensure High Quality Streaming is selected in Preferences > Playback.
Log in using a Free account.
Navigate to Preferences > Playback. The High Quality Streaming option will be ticked, unticking the box will allow the action and also give you a "Premium Feature" warning. Once unticked you can not reselect the box.
[Meredith edit] This is a graphical bug. The stream is not being played at High Quality no free accounts. When we have an update on this cosmetic fix we'll let you know. Thanks!