No idea. If the pigment is not removed, its discoloration should not change. Selectively removing lead from the pigment can be difficult (although it doesn't have to be impossible, a lot depends on the exact composition of all substances involved).
Right, maybe it has to be restored with square mm's at the time, which may take months if not years. A bit weird considering that Vincent van Gogh painted oh several dozens paintings a month with a peak of 80, the month before his death