I was told by a Viking rep that there are a couple of caveats in terms of getting promos added to an existing reservation:
-if you already paid in full, no promo will be honored after that point
-if you haven’t paid in full, they will apply a valid promo IF there is still availability in the class of cabin you have reserved.
best way to think of it is that they “refund” your reservation and make a “new” reservation under the promo. Hope that helps