In a method of operating a machine processing printing material sheets and having a sheet transport drum in various operating modes, drum circumferential shells are kept fixed on the sheet transport drum in an outer position in a first of the operating modes and in an inner position in a second of the operating modes. During a drum conversion to be carried out before the start of a third of the operating modes, the drum circumferential shells are removed from the sheet transport drum and, in this third operating mode, the sheet transport drum is rotated without the drum circumferential shells.