Hi rjd, the renewal process (including upgrade) is supposed to work just like you outlined, i.e. the days left of your old license should be added to the new one. Apparently an error occurred during the automatic process. Sorry for that, we’ll investigate it and fix the problem asap.
Meanwhile please drop us a ticket at https://support.avast.com/ or send an email to support@avast.com with the details suggested by .: Mac :. We’ll prolong your new license individually.