A laboratory-scale two-stage swirling burner fueled with dodecane is studied experimentally with the help of high speed (20 kHz) spray PIV and chemiluminescence imaging. For a lean operating point at 73 kW, fuel staging is changed and a hysteresis cycle is highlighted. The flame is first lifted from the injector exit plane and exhibits a strong thermo-acoustic instability. The transition to a more stable state is studied by changing the staging parameter while keeping the power constant. Thanks to a well-controlled environment, several runs are performed to ensure that the phenomena observed during these transitions are reproducible. In less than 10 ms, flashback happens in the inner recirculation zone and the flame attaches itself to the injector in a tulip-like shape. The spray then presents a wider angle. Thermo-acoustic instabilities are barely detected and the flame exhibits a strong response to the precessing vortex core. The flame finally widens into a more common V-shaped flame, either in less than 50 ms (for half of the experiments) or after more than 500 ms.