ppm/K to ppm/°C formula

Kelvin and Celsius are the same scale, but Kelvin is shifted so zero lines up with absolute zero while Celsius is shifted so zero lines up with the freezing point of water, for convenience. The step size of both are the same.

It is like how km can be used to measure distance from my hometown instead of your hometown to the same location. The size of each km is the same, but your reference point is different.

The difference is 273.15 K or °C. So 0 °C is 273.15 K, but that is absolute temperature. A ppm/K or ppm/C is a relative measurement that only cares about changes in temperature and in that case the 273.15 term appears twice, once for K and once for °C so it subtracts itself out of the equation.

Yes it is true because a change of 1 °C is the same as a change of 1 K.

I was searching for the formula and all I could find was that 100 ppm/K = 100 ppm/°C.

Remember that the value that's changing is the measurement - resistance, voltage, current, intensity, length, etc. For example, the calibration of a range sensor may vary by 100 ppm/K which means that the distance measured may vary by 100 ppm (0.01%) for every one kelvin change.

Is this true?

Since the kelvin unit is the same as 1°C the statement is true.