Helicopters maintain a flight attitude by providing lift through the high-speed rotation of metal rotor blades. Therefore, it can be considered that as long as the rotor rotates normally, the aircraft can fly after flipping up and down. But in fact, most commercial*** can't sustain a sustained upside-down flight. The simple reason is that they weren't designed for that.
In order to fly upside down for a few seconds, the joint connecting the rotor blades to the fuselage had to be redesigned and reinforced so that it could withstand the belly-facing load. In addition, the engine needs to be overhauled so that the fuel and lubricant can be delivered exactly to the right position during the fly-up. The flight controls also need to be adjusted so that the blades can be reversed downwards.
Even if it can fly backwards after being specially designed, it needs a professional pilot to control. Because when flying upside down, it takes some professional skills to control the aircraft, such as to make the bottom of the aircraft (the top when it is not flipped) generate lift, so that the aircraft can maintain a flight attitude, and the rotor blades need to be tilted slightly downward. In addition, the controls are reversed, so the pilot needs to reduce the thrust to gain altitude and push the rod back to move forward.