Green Bay winters are long and cold: the 1991-2020 January normal is a high of 25.5, a low of 11.1 and a mean of 18.3 degrees Fahrenheit. When a furnace or boiler fails in that weather, a frozen pipe can follow quickly, so a slow response matters. The City of Green Bay requires an HVAC permit whenever heating work is done, including furnace and air conditioning replacements, and it inspects the work when it is finished. If you rent, a licensed contractor must do the work, and a licensed HVAC contractor you hire submits the permit. Statewide, Wisconsin Code SPS 305.70 says anyone who installs or services heating, ventilating or air conditioning equipment must hold a registration as a registered HVAC Contractor, with some exceptions. Before you hire, ask for the contractor's state registration, ask who will pull the city permit, and ask for the scope of the repair or replacement in writing before work begins.
Sources: National Weather Service Green Bay (source, opens in a new tab), Wisconsin Department of Safety and Professional Services (source, opens in a new tab)