They offer a $15 'upgrade' to let you spend several hours in the other
side of Universal before going over to do Halloween Horror Hights. You
can enter early and do Islands of Adventure then cross over to the area
where the Halloween stuff is.
Thing is, that once you have bought it, your 'upgrade' to your multi-day
ticket is not an upgrade. If you buy your ticket online and print it
at home, it comes out as an 'add-on'. It is a separate ticket, and it
only works once, one one day only. Event though the ticket you tried to
upgrade is a multi-day ticket.
Shame on universal for bait and switch. Why advertise it as improving
the ticket you buy, when really its nothing to do with your multiday
ticket.