> could a library author claim their old code infringed on copyright to force users to upgrade to a paid version? Anybody can claim anything. Only the holder o

> could a library author claim their old code infringed on copyright to force users to upgrade to a paid version? Anybody can claim anything. Only the holder of the infringed copyright has standing to sue, so the library author in your example has no power to force anything. They may also be civilly liable for damages to the library users and, if unlicensed code was planted deliberately, then also criminally (extortion).