From what I understand there is no issue adding a day during your trip Undercover Tourist tickets, correct?
Is it the same as Disney where you have to enter the park, but not have used up the whole ticket? Does the "bridge"/upgrade price work the same way? You're only paying the different between the Universal sold ticket price, not your discount price?
UT is offering a 2 day Universal Ticket with a day free. That doesn't complicate anything does it?