Book now to move later?

[Aerin75]

Looking for some thoughts for my situation. Thanks in advance for anything - may not be able to check replies for a bit.

We are planning our cruise for Fall Break 2017; we get a two week one from 10/2-10/13*. We want to use this trip to mark off a bucket list item and do a couple of nights at GF before the cruise; we will be driving down from the Midwest.
Based on calendar for 16/17 school year

I have two options: the 9/30/17 7 night Western on Fantasy, and what I hope will be a 10/7/17 Eastern on Fantasy. We really want (and looking at historical trends had planned on doing) the 10/7 because that's the second week of the break.
Do I go ahead and book the 9/30 and move it when the Fall itineraries come out - on the off chance the 10/7 doesn't happen/becomes a 10 night Southern or something like that?
If we have to do the 9/30, we do need to take the snot....I mean lovely pre-teen daughter....out of school for a couple of days, but that's not the end of the world, and it's why we want to do the 10/7.

Am I just over-analyzing? Feel free to say yes - it's my habit!

Thanks so much!!