When you call for the Fantasmic Package at the 90 day window, you will be given a choice of very early times. Take the latest one.
At the 30 day window, call again, and you should be able to get a later time. If you can't, call in about a week and keep trying until the day before you go. You should be able to get the later time to better suit your schedule.
Although Disney may automatically cancel your first time, make sure they are aware of it just in case.
As I've said before, I think Disney gives the very early times since they may be the hardest to fill ordinarily. Since you are requesting the Fantasmic Package, Disney has the leverage. Apparently, only when Disney feels that the later times may not be filled will they give you a later time (usually 30 days or less before the appointed date).
Good luck, and have a great time!