'Tricking' the App Store to allow re-downloading a purchase

Question

Hopefully short version of a rather long ordeal:
An app I use recently became available only on the MAS. After paying and downloading it, I discovered that the old version was still installed as a Pref Pane. The developer's site suggested using an uninstaller but warned that it might also remove the new version and one would need to re-download.
What actually happened was very unpleasant and frustrating. The new version did seem to disappear, but only from the Applications folder. Bits and pieces of both the old and new versions remained in various places on my drive. Unfortunately, they also appeared on my external backdrive. These remaining resources seemed to make the MAS think every thing was fine and I should not try re-downloading the app.
The short story: I deleted every instance of any resource for both the new and old versions of the app. Emptied the Trash. Shutdown for the night.
This morning, I disconnected the external drive before starting up. Opened the MAS and was surprised to see every single purchase listed as simply "Install"! Note the lack of the "ed"! MAS was saying I had purchased these apps but that I could now install all of them! I didn't push my luck any further than re-downloading the one app that started this journey!
Things went totally as expected and the app is now successfully running. Finally!
Moral? If you can't get the 're-download' to function, simply do your best to delete and trash any and every resource that app placed on any of your drives, including externals. It may also help to block Spotlight from icluding those backup volumes from its index.
I'm still not sure exactly what the MAS uses to know what is installed, but it obviously reads some info on the drives. This info seems to be the actual app andor any of its resources (inluding plists). I did find what appears to be a new method of recording installs. In previous versions, OS X stored ".pkg" files in your "/Library/Receipts" folder. In Lion, there are a few of those files still there, but there is now a "InstallHistory.plist" file listing at least Software Update and MAS instals. However, its presence doesn't seem to be used by MAS in determining what you may have purchsed. Just an interesting bit of trivia? ;-)
Others have stated that using the 'option' key will allow re-downloading a purchase. That has never worked for me, however. The above method has and the disconnecting of external backups may be critical, also. So, my three-step method for the future will be:
1. Disconnect external drive(s).
2. Completely delete any and all resources of the particular app and empty the Trash.
3. Restart and open the MAS expecting to see "Install" labels and buttons. ;-)
Hope this will save some frustration for others...

Answer 1

Hello -Tuck-,
Thanks for using Apple Support Communities.
The apps prompting you to sign in with a different Apple ID when updating were purchased with that Apple ID.  You will need to sign in with the Apple ID used to purchase or accept the applications before you can update them.
Alerts may occur when accepting bundled applications from the Mac App Store
http://support.apple.com/kb/TS4373
Take care,
Alex H.