ADS-B, typically, does not outright replace your Mode C transponder but is an addition to it.
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For aircraft operating at or above FL180 (18,000 feet), you must be equipped with a Mode S-transponder-based ADS-B transmitter. For aircraft operating below 18,000 feet and within U.S. airspace, you must be equipped with either a Mode S transponder with Extended Squitter or a Universal Access Transceiver (UAT).
While you don't need an operable transponder to fly below a Class C shelf, you will need one to fly above Class C airspace. As you approach a Class C airport, you'll contact that airspace's approach control. Call ATC on the radio before you're in Class C airspace and make sure to tell them: Your position.
Pilots can easily make a misstep by turning off their transponder, and there are very limited circumstances which allow a pilot to do so.” Civilian aircraft can turn off their ADS-B Out only if specifically instructed to do so by ATC, or if they are the non-lead aircraft in a formation flight, Duke said.
You may fly without ADSB under the Class B or Class C shelf provided you remain outside of any Mode C veil. §91.225 tells you where you cannot fly. It says you cannot fly within the lateral boundaries of the Class B or C airspace or above the airspace.