Originally Posted by
asadliv
You can do it online
here
Yes, you need 2 e-vouchers. Y, B and S can be upgraded. There usually need to be availability in the award inventory 'I'.
Are you sure about S? "... Turkish Airlines (TK), US Airways (US) and United (UA) are possible from Economy booking classes Y and B." (from the page you linked to) Is there any place where I can see that S is eligible as well?