You could configure the batteries in banks so that various series and paralell combinations could be selected.
You could also vary the field current to a shunt connected motor. Reducing the field current is very much like changing gearing. When field current is reduced, speed increases, but torque is reduced. Since field current is modest compared to armature current, it would be practical to control this either with battery bank taps, or variable or tapped resistance.
Alternativly, two field windings could give 3 speeds when supplied from a fixed battery voltage:
1-Low n * I winding Highest speed
2-High n * I winding medium speed
3-Both windings in paralell (or maybe series) …lowest speed.
The effect of series connection depends on the winding turns and resistance. It might be useful as a fourth speed.