I say this with a caveat, because the rules change a lot...but my planner has said that if we're unable to do our MK shoot in November when we are hoping to (our wedding is Sunday November 16, we are hoping for MK shoot on 11/18) that we can come back anytime in the future and do it, as long as there is availability (and we are willing to pay the money). I had originally heard within 1 year, but at the moment "anytime" seems to be what they are saying.
We won't be able to reserve our date until 45 days out because we are not using Disney photography for our wedding...and even then, even if we are told we get the date we are hoping for, it is my understanding that the park can cancel it (and often does) due to pretty much any reason they want to. So it's definitely a risk...but I really want those pictures as I am a castle obsessed girl, so we're going to try. We're DVC so if it doesn't work out, I'm relatively confident we could do it on another future trip, but the dress and tux will be a bit of a pain in the rear. Fingers crossed that it somehow magically works out during our trip...
Good luck!
Brooke