What does a scale changer do?

A scale changer on a harmonium moves the keyboard sideways relative to the reeds, so the player can change key while keeping the same fingering. Banglapedia lists scale-change harmoniums as a type, alongside coupler, box, single-bellows, double-bellows and English-bellows models. It suits the Indian harmonium, whose keyboard is fixed to the 12 equal-tempered semitones.