No, they won't change an existing ressie to give you the 10% off. However, you can book now. Then, when you are on board, you can check the prices and IF it is the same price or cheaper, re-book while on board. As soon as you are home, cancel your first reservation (if you are still within the full refund time).
The only downside to this is that you need to be able to tie up the deposit amount for a month or so until they manage to get your refund sent to you.
On the positive side, early booking savings are better the farther out you book...so booking now might give you a better rate than you can get while on board in January, even with the discount. Best bet...book it now. If you can get a better deal on board, book then too and cancel previous. If you can't get a better deal on board, you still have your first ressie.