Actually this response does nothing about a much larger issue on the App Store.
Take the Tweetbot issue which I had hit me. They had released a new version, Tweetbot 2.1.1, right before this issue happened on Oct 15. This updated version is incompatible with Mac OS Mavericks (10.09) so those running Mavericks were stuck with the previous version. This means that for all those running an OS older than 10.10, you're only able to run Tweetbot 1.6.2. If you go to the App Store and try to update it, or even re-download on such an older OS it explicitly tells you of the incompatibility and says that it will download the "old" version for compatibility.
That would be fine, except the old versions are still signed with the EXPIRED CERTIFICATE! So even if you follow the directions to "re-download the damaged app" it will refuse to run because the certificate signed is expired. So the only "fix" is to upgrade your OS to 10.10. Sure it can be argued to upgrade to the latest version, but there are quite a few instances where this is impossible to do and as such, Apple has just put an expiration date on older software preventing you from running it by linking it to this certificate and not providing developers a way to re-sign those submissions with an updated cert. Neither does the App Store itself provide such a facility.
So if by the current expiration date which I believe now is 2 years from now, if your computer is unable to be upgraded to current OS and the current version is unsupported on your system, then you are completely out of luck and stuck with no app that you paid for. This makes the "download old version" feature in the App Store completely flawed if they provide no way to back sign older provided version on the store front.