You can certainly make the request. They will have a table set up for just such requests and will do what they can to accomodate it. However, that is the more popular seating so they won't guarantee.
The time and place to make dining changes will be listed in your Navigator. On the Magic in January it was outside Sessions on Deck 3. We went around 2:00 pm but I think they open earlier.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!