I agree, I would not change my plans for this small number of extra people. I would not think they would be making dining reservations at this late time, and not all of them will arrive right at 6pm. Heck, if I won these tickets, I would wait until 9pm when all the rides will be walk on, versus fighting the full crowds from 6-9pm. They also won’t have LLMP, presumably, so if you have it, you could just plan to book some LLs during this time and avoid the extra folks. Honestly, I would not stress yourself out to change your park days over 2500 potential people over a 3 hour period, of your entire park day. But I totally understand your frustration!! I would feel the same in your shoes!!