I have the same story - bought Sketch 2 in France for 50 euros, moved to US as they updated to version 3. Can't get the old version back (it just doesn't appear in itunes at all - tried to get this resolved with no luck) and my only option is to re-purchase the product.
My understanding is that for both the Mac and iOS app stores, the Purchases tab has all the apps you've bought, whether they're currently available on the store or not. This is how I re-download older operating systems or previous releases of apps. That said, I can imagine Apple also having a parallel system that takes the app away from there too -- but I don't think this is the default.
