Hi, Stacey! You shouldn't miss anything too important, as most of the bigger festivities - like the parade and the fireworks - have typically begun around the 8:00 PM time frame - with a repeat parade later in the evening (only one fireworks show, though).
What my family has always done is used the Mickey's Not So Scary Halloween Party ticket to gain entrance earlier. You have always been able to arrive at 4:00 PM with your party ticket. I make my dining reservations for 4:45 or 5:00, and that way, we are ready to start trick or treating and enjoying the party as soon as things are starting.
If you can't change your advanced dining reservation, don't worry. The evening will still be packed full with fun and activities, and having your dinner last into the early part of the party shouldn't do anything to diminish your fun.
I hope you have a wonderful time! This is one of my family's favorite events!