There are right ways and wrong ways to do things. And in between, there are a variety of not very good ways.
Traditionally duck confit was reheated very gently as in a cassoulet; or picked off the bone and served at room temperature, as in a salad.
Duck skin which has undergone the confit process is flabby. There's very little you can do to crisp it up. Shallow frying -- the technical term for what Someday described -- is marginally useful. Flash, deep frying is somewhat better, and so is a torch. But, because it was already cooked there's only so far you can take it.
Searing... From a purely technical standpoint I'm not sure you can sear something which is already cooked through. In any case, you cannot get a meaningful Maillard reaction going -- which is, after all, the point of searing.
Heating in a 350F oven... Sorry. No. You just went through two and a half hours to poach the duck low and slow so it will be tender. What do you think will happen if you heat it through at high heat? Tough and dry. Tough and dry will be the result if the duck is heated through by any too-hot method. The different sorts of frying and the torch -- won't do nearly as much harm as long as you're only heating the surface. But as I said above, there are limits to how far you can go to give the skin from a confit much texture.
Because smoking is also low and slow, the skin from smoked poultry is similar to that which was confited. The skin of this chicken, which was smoked at around 250F, was light brown and too pliable when the chicken came out. I used a torch to give it some color and bring it back. You can see how the skin tightened so much it ripped in several places, but -- while the skin was palatable -- it still wasn't what you'd call crisp.
The skin on that particular chicken was highly seasoned, and the seasoning was intended as an integral part of the dish. But the ducks one confits are not seasoned in the same way as traditional confit is intended to take on the flavors of the dish to which it contributes. From a flavor and seasoning standpoint, the skin is "nothing special." And -- not to belabor the point -- it's very difficult to make it so.
Although I don't have any duck pictures handy, I don't know how many confit of ducks I've done over the years, certainly more than a hundred; and I've presented them in dozens of ways. Take it for what it's worth.