Not sure about how to gift an upgrade but I've seen it happen so it must be possible.
Found this:
http://www.chess.com/home/gift_subscription.html
Questions for staff here:
http://support.chess.com/index.php?_m=tickets&_a=submit
(Check "Billing Help & Premium Memberships", fill in the form).
Is there a way to give someone an upgrade as a gift?
For example, if someone has a year long gold membership is there a way to buy him a diamond membership for the rest of that year membership?