I hadn't looked into this detail, but the only sensible way to do this is the same way as the government scrappage scheme a few years ago where only a current registered keeper who had owned the car for 6 months was entitled to anything. (I think VAG secretly hopes that word will get around that the NOx fix ruins the performance of the cars and owners will stop taking their cars in for it. That doesn't help people with respiratory problems from breathing the excessively NOxious fumes, but prior to 2009 everyone was breathing NOxious junk from all diesel engines anyway.)