Yes, as long as you are not booked in Concierge (which have different rules) you can switch your cruise date without penalty if you do it before the PIF date. If you are changing dates, the PP is correct, the fare for your cruise would be the current price on the day you book that date, not the date you made the initial deposit.
As far as when
DCL would release schedules, that's always a guessing game on here. Part of that depends on what time of year you are looking to cruise. The last few years, DCL has announced schedules in phases so the first part of 2018 (through about May) may be released in Fall 2016; then the rest of the year may be out in Spring 2017. But this is a rough guess, DCL keeps changing when they release schedules.
One thought, if you aren't sure when you will be cruising or are looking more for 2018 dates, have you considered opening a Disney Vacation Account? It was created to give you a way to pay for your Disney trips even before you book and you may be able to earn gift cards for extra savings on your cruise.