I saw lots of topic on this forum about people who can't play tracks anymore. When they check the preferences, High quality Streaming (or Extreme quality) is enabled. When they disable High Quality Streaming (or set Extreme Quality to High or Normal) everything is working again.
When a customer gets downgraded to an free subscription, reset the sound quality preference to the default one automatically, not waiting for the customer to do it, this way you can prevent it from tracks that are unavailable due this issue.
Since this issue has been confirmed as bug over here I think it's best to wait for a fix from the developers. Therefore it's not really a "feature" request but just fixing a bug and it's probably the best to focus on the bug topic. Any news regarding that problem will be announced there. ;)