If you're talking eg about frankish states, then yes – they doubtlessly existed. But e.g. more to the east „countries” as we know it didn't really exist at that point. To give you example: unification of Poland happened circa X century AC. Prior to that point, there were no „country” here: it was all about local tribes, which aren't really documented. You can't really talk about eg Poland and Bohemy at this point, as there were none: there were only some tribes, each acting as it was the highest political entity of whichever land there were in. And some of them migrated, so you can't even „safely” say where they were at which point.
I imagine you could do some game with „countries” model, but it would be limited to Byzantium, state of Clovis etc, as in good half of CK II map there were no countries during the time period. There, „migration” wasn't exactly only demographic factor, as migrating tribes were major political entities in absence of any else political presence. OFC, I am not talking about Balkans ATM, as I am versed somewhat only in Byzantine history
