Passing from nozzle diameter 0.4 mm to 0.2 mm causing filament jamming in heater
0.2 mm and 0.4 mm are half the diameter, but the maximum flow is not just half: Flow scales with the area. The 0.4 mm nozzle has an area 4 times as the 0.2 mm one:
$\frac{A_1} {A_2}=\frac {0.2^2}{0.1^2}=4$
You need to reduce print speed or the volumetric flow by this factor or make sure your printer can handle the increased flow by reducing the viscosity of the melt - for example by increasing the print temperature.
Also note, that a 0.2 mm nozzle can't be operated with layer heights above 0.15 mm.