It's one of those days where I don't see what I'm looking for, and I have that unshakable feeling that it is so obvious that I will feel foolish once it's clear to me. What better place to ask for help than Live Journal, where a very limited number of people are listening, and they are already aware of how foolish I am!
I was checking on our memberships to next year's WorldCon, and found that the kid has only a Supporting Membership. Now, I'm sure he didn't vote in Site Selection, because he was a Child then, but he's now old enough to be a full Adult.
Their website, http://www.laconiv.com/2006/reg/reg.htm#chart, lists the conversion price for people who voted and, on a different page, the costs of an Attending Membership and a Supporting Membership.
Is the conversion from Supporting to Attending just the one minus the other? Or something else? And I don't see a handy Paypal option for that, even though all the others are there.