This could affect a lot of people. We're out of our home country and I was recommending a free app, by email, to a friend. Getting a link, I went into the App Store and called it up. Oh, it's been updated. May as well download and install. Well it *seemed* to have done so, but of course it wanted me to sign in to install it. You cannot sign up from another country without re-registering and all the details relating to your move.
The update had deactivated the original one - the original app is still in the Applications foldere, but won't launch. So it seems the lesson is...
Save updates until you get home!