Bring two quarts salted water to a boil. Remove from heat, add the rice noodles, and let soak until they're just tender, 5 to 10 minutes (or follow the instructions on the package). Drain the noodles well and set on a plate lined with paper towels.
Cut the celery into 4-inch pieces and then slice lengthwise into thin strips. Use a vegetable peeler to shave the carrot into thin strips. Holding your knife parallel to the cutting board, slice the chicken breasts in half (to create thin cutlets) and then cut crosswise into long, thin strips.
Season the chicken with 1/2 tsp. salt and a few grinds of pepper.
Combine the curry powder, sugar, and 1/2 tsp. salt in a small bowl and set aside. Heat 2 Tbs. of the oil in a large skillet or wok over medium-high heat until shimmering. Add the chicken to the skillet and stir-fry until the chicken has lost all of its raw color, about 2 minutes. Transfer the chicken to a bowl or plate. Heat the remaining 3 Tbs. oil in the skillet and add the scallions whites and ginger; stir-fry for 30 seconds. Add the celery, carrot, scallion greens, and curry mix, and stir-fry for 30 seconds to soften slightly. Add the drained noodles and the chicken and toss until the noodles have turned an even pale yellow and are hot throughout, about 2 minutes (scrape the bottom of the pan if necessary). Toss with the soy sauce, taste for seasoning, and serve immediately.