Heat the oven to 425°F and toss the potatoes with the olive oil, 1 teaspoon salt, paprika and black pepper in a 9-by-13-inch dish.
Roast the potatoes until browned at the edges and nearly tender, 30 to 35 minutes, stirring once.
Whisk the sour cream, milk and remaining salt and fold it with the cooked chicken and hot potatoes.
Top the casserole with cheddar and bacon and return it to the oven.
Bake until the cheese bubbles and the potatoes are fully tender, 12 to 15 minutes.
Scatter the scallions over the top and rest for 5 minutes before serving.