Nelson said that after reviewing the matter, he felt a charge of criminal homicide was more appropriate in this case.
He would not elaborate.
Traini had been held at the Schaffner Youth Center in Harrisburg, Pa., on the earlier charge.
Nace, of 1 Zion Road, in Newburg, Cumberland County, was found dead Nov. 5 in a Franklin County trailer park at 7500 Molly Pitcher Highway.
A Pennsylvania State Police trooper who responded to the scene at 6:35 p.m. found Nace's body in the driver's seat of his mother's 1995 Saturn, which was parked at Lot No. 62, according to the affidavit of probable cause filed in Weyman's office.
Trooper Roger E. Hall also saw a .22-caliber rifle laying barrel-down on the front passenger's seat of the car, court records state.
Traini discussed his role in the shooting during an interview with state police at 12:04 a.m. Nov. 6, according to allegations contained in court records.
He said he intentionally caused the death of Nace and that Nace had asked him to help him commit suicide in the car, court records allege.
According to allegations in the charging documents, Nace asked Traini to hand him the rifle, and Nace put the barrel of the rifle into his mouth and said to Traini, "OK, I'm ready."
Traini pulled the trigger, court records allege.
Chief Deputy Coroner Jeffrey Conner said an autopsy showed that Nace died of a single gunshot wound to the mouth.
Nace was a junior at Shippensburg Area Senior High School and a second-year masonry student at Franklin County Career and Technology Center.
Police spent last week interviewing students and friends at the career and technology center.
Those who knew Nace described him last week as a good student who was popular and well-liked.
More than 50 students sought counseling provided by the school to help them deal with Nace's death, said Dalton Paul, executive director of the career and technology center.