Method for switching over a combustion device of a gas turbine engine from operation with a first premixed fuel to a second premixed fuel

The method for switching over a combustion device, for example a burner of a gas turbine engine, from operation with a first premix fuel (10) to a second premix fuel (11) comprises reducing and stopping a first premix fuel supply and then starting a second premix fuel supply. In an intermediate phase (5), after the first premix fuel supply stops and before the second premix fuel supply starts, the combustion device is operated with one or more pilot fuels (12, 13) generating only diffusion flames.