The thing to keep in mind is that September (After Labor Day) is really quite empty. Everybody is back in school, and major refurbs are done. Sometimes Believe is shown on Sundays, but it isn't a certain thing. If this is a make or break part of your September trip, then perhaps you should consider changing your dates, but I don't know that I would reccomend it per se. You see...in September, the Santa Ana Winds pick up, so many times the Fireworks are cancelled, even if scheduled, so it wouldn't be certain. This can be the case for Fantasmic as well. (I remember 2 years ago during September, Fantasmic was restarted twice, because the winds were too strong, so they stopped, and then they restarted when it calmed, but it picked up again - They never finished a full show that night). Also, because of the light crowds, then you should be aware that many attractions will be refurbing, so don't get your heart set on something.
Currently the September refurb list stands at the following:
Casey Jr. Circus Train
King Arthur Carrousel
Matterhorn Bobsleds
Sleeping Beauty Castle
Storybook Land Canal Boats

e:
Hopefully everything will be ok for you
~Tina