Any change in the Eastern itinerary may only be for the summer, so it should not affect December sailings.
As for the Christmas sailing, who knows? Christmas falls on a Saturday that year and there was talk of doing something to change the departure date for that sailing, as they really don't want sailings that return to Port Canaveral and leave Port Canaveral on Christmas Day. No one wants to be traveling to or from the ship on that day and no one wants to be away from their family to check people in and off the ship on Christmas either.
Since the Wonder departs on Sundays, my guess would be that you will see a 6 night sailing the week before Christmas (that one was supposed to be a Western) and an eight night sailing Christmas week. The Christmas sailing would leave Friday, December 24 (that one was supposed to be Eastern). Itineraries would have to be different those weeks because of time constraints.
I don't believe you have any chance of getting the $200 onboard booking credit retroactively. If you have this problem again, the way to get around it is to pick a sailing that is open and then change the date (in the same reservation) once the sailing you really want opens. Last time I was on, that's what they were telling people and that's what we have advised some of our people to do as well. It's always subject to change, but in the past, they were allowing for one date change before the $200 credit dropped off.
Good luck! There is no better way to spend Christmas!