One of my Macs is stuck on Snow Leopard. On Lion & El Capitan, I sign in to the App Store again and it's resolved. But on Snow Leopard, it doesn't display the sign-in error like 10.7 - 10.11 do. It just complains and tells you to redownload the apps from the App Store.
But I can't. Apple doesn't let you download old versions from the Mac App Store. Byword 1.5.2 is the last version that works on 10.6, the current 2.x version requires 10.8. Similarly with Tweetbot, Mémoires, Space Gremlin, etc. Unless Apple magically allows old downloads, I don't think this can be fixed without Apple patching Snow Leopard. I'm not going to hold my breath.
It really seems like Apple has permanently broken my purchased apps.