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.
UPDATE: OK, this looks more complicated than I thought. It seems that free apps can update older versions of themselves in this way, but the same rules don’t apply to paid-for apps. Crucially, you will only get automatic free updates for apps that you have bought via the App Store – even if you already own a copy of the same app that you bought outside the App Store.
What I’m still not clear about is what will happen if you own a pre-Store copy of a paid-for app, then buy it again from the Store. Do you end up with two copies? I’ll do some more research and experiments, and post an update in the next day or so.
Thanks to Chrispian and others in the comments for highlighting these additional complexities. If you’ve made discoveries of your own, do pipe up in the comments box.