We went to the ticket booth to pick up our park hopper passes (we bought through getawaytoday.com but had to pick up our passes at the ticket booth). We asked to upgrade at that point but we were told to start using our park passes & to upgrade later on in the week after we started using our park passes (not sure why we just couldn't upgrade then). On day 6, we went again to the ticket booth and were able to upgrade for just the $5/person.
Thanks. We'll probably check first too and if needed go back later on day one or first thing on day two. I'm looking for schedule flexibility so waiting until day six is not going to work.