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I have just had a reply from Kaspersky, a very polite and helpful one at that. They have told me that my 2012 edition is now outdated and no longer supported. Seems logical, although why it wasn't "outdated" in 2013 and 2014, and in April this year, when I renewed my subscription is beyond me.
They have sent me detailed instruction on how to uninstall my 2012 edition and reinstall the 2015 one.
But if they had sent me an email, or incorporated a proper error message when I tried to update the databases 2 days ago, I wouldn't have had to put a request in for help. They must have millions of people like me that bought the product in 2012, have been renewing my subscription every year, but found themselves in this problem when the system automatically tried to update a few days ago.
They have been very prompt and helpful with advice but they could have saved themselves no end of bother if they had notified me in the first place. Doesn't seem to me to be the right way to run a business !