That would be a epic fustercluck. That would require that everybody for HHN be given a wrist band as they enter the park.
So either you have to have the gate people put down their scanner after each time they use it to put a wrist band on each person, probably at least tripling the time spent for guest who knows what they're doing (there's no hope for the morons who are SHOCKED they need to show their tickets after waiting in line seeing every single person in front of them use their tickets and have to go digging through purses and backpacks to find them).
Or, you have to give each person a wristband and tell them to put it on themselves which is always a disaster. I go to local races a lot and you pay for your entry, they give you wrist band and you enter...and every single person stops the second they walk through the gate to put it on and just cause a massive bottleneck and it's just dozens of people. Having thousands of people trying to enter the parks at 6:30 and all wanting to go into the first house 100 feet into the park and needing to have a wrist band to enter the line...you're just asking for a disaster. With a lot of people "pregaming" and already having their drink on when they get there just makes it an even worse situation.
Then you'd also have the potential situation of a family with a double wide stroller trying to "go against the flow" and get out of the park...with scary monsters scaring the kids.
HHN is such a different beast from day time operations that a "gradual changeover" spilling into the event isn't practical.