DOVER — Prosecutors have upgraded the charges against the man who allegedly assaulted a Frisbie Memorial Hospital security guard last week, now that the guard has died due to injuries he sustained. Tyler Thurston, 29, of New Durham, now faces a felony manslaughter charge in addition to the felony second-degree assault previously filed against him for allegedly attacking Richard Semo, Strafford County Attorney Tom Velardi announced Monday. Semo, 64, of Farmington, died Friday night from injuries he sustained in an attack in the parking lot of the hospital around 3:20 a.m. on Sunday, Dec. 13, Rochester police Capt. Todd Pinkham said over the weekend.