Can I update apps that I've already purchased through the Mac App Store?

From Daring Fireball:

Panic on the Mac App Store

Cabel Sasser on what it’s like for existing Mac apps that are now in the App Store:

PS: If you’re a current customer, there’s no way to convert a previous purchase into a Mac App Store purchase — that requires a re-buy. But remember, what you have now will continue to work just fine.

UPDATE: The Mac App Store may show software bought from us previously as “Installed”, even though they’re two different licenses. You will not get Mac App Store auto-updates unless you purchase from the Mac App Store. To re-enable the “Purchase” button in the Mac App Store, just drag the app to the trash. Your preferences/sites will not be affected.

In other words, for apps that use the same bundle ID for the both the Mac App Store and non-App Store versions, the Mac App Store will not install updates to already-installed non-App Store versions, but it will recognize them as being already installed. If you want to get updates via the Mac App Store, you’ll have to re-buy the app through the App Store.

It depends.

As reported in the Daring Fireball link: it doesn't work for some apps like Panic's Transmit, Unison or Coda. The licensing schemes used when you buy via the App Store versus directly from the developer are different and won't allow for it.

But this article from Cult of Mac has the author successfully updating a previously installed version of TextWrangler to the latest version via the App Store.

I just tried a quick Mac App Store experiment.

One of the more tempting offerings on offer there is TextWrangler, the excellent all-purpose text editor from Bare Bones software. It’s been free on the web for years, and now it’s free on the store.

I already had a copy installed, but on closer inspection, my existing version was 3.1, and the one in the App Store was 3.5. What would happen, I wondered, if I clicked the “Install” button? Would the App Store version be installed separately? Would there be some kind of conflict? Or would it Just Work?

Of course, it Just Worked. TextWrangler 3.5 was installed where I expected it to be, and the older version was overwritten.

So now you know: if you see something in the store that’s already on your hard disk, you shouldn’t have any problems installing a newer version from there.

I disagree with the Cult of Mac articles conclusion, that "if you see something in the store that’s already on your hard disk, you shouldn’t have any problems installing a newer version from there" -- as Panic has directly contradicted that conclusion and they build and sell through the App Store so, pretty authoritative there.

I did talk to the developer of my favourite Twitter app, Kiwi, and he said he wasn't sure if the App Store could be used to update copies of Kiwi bought before the App Store came in to existence.

So welcome to the Land of Confusion.