I called Universal yesterday with this exact question. We are buying a 3 day through
UT, but we might want to add a 4th day when we are there. Universal told me (on the phone) that we would just have to pay the difference between the 3rd and 4th day ticket price and NOT have to pay the difference between the Universal price and the UT price. Did they give me wrong info or has the policy maybe changed?
Thanks!