Can a healed stress fracture be diagnosed as more serious fracture on x-ray when a healed fracture is found?

They are same: A stress fracture is just a type fracture. Only difference is it occurs with chronic repetitive stress instead of a single episode of trauma. They both heal the same. Bony callous will form. If the stress continues, the area can refracture, and more callous will form. Therefore, a stress fracture is just as serious, especially if you have not identified cause.

Healed fractures: Usually look alike unless there is deformity of the bone at the fracture site suggesting healing of an angulated or displaced fracture. So, a stress fracture that is healed will most often look like a post traumatic healed fracture.