You can absolutely adjust the current for a stepper motor. Lower currents use less power, but have less torque, leading to an increased chance of missed steps. In a system like this, the motor torque becomes a linear force to move a belt — and the force required to move the belt depends on if a piece is moving with it, and which piece. It’s perfectly reasonable to bump up motor current when moving a piece, and bump it down when moving just the belt to grab the next piece.