If you don't say anything to the CM at the booth, but just show your tickets, they "should" be treated just like regular full priced tickets. This is what people here have reported in the past with upgrading LMT tickets to APs and other things. The CM at the booth cannot know from looking at the ticket itself that you bought it at LMT, so don't say anything about a discount price. Be prepared to pay more than $15 per ticket to add a day, but don't be surprised if that is what you are charged. The tickets you have in your hand look just like regular tickets. Pixie dust happens! By the way, hope your trip is going well and that you all are having a magical time!