A five-year-old dispute over the taxable value of the pulp mill in Old Town is headed to Maine’s highest court, with the city planning to appeal a recent ruling by the Maine Superior Court in favor of a former mill owner that has sought a lower property value. If the Maine Supreme Judicial court upholds the June 30 decision by Superior Court Justice William Anderson, the city will owe the former mill owner, Expera Specialty Solutions, approximately $600,000, according to Travis Roy, Old Town’s assessor.