I would like to know in what ways and why the concept, appearance, and manner of the zombie has changed from its original conception (early 20th century) to today's?
The silent film The Cabinet of Dr. Caligari from 1920 is thought to be the first depiction of zombies in film. While it doesn't feature the walking dead. The story is about the fantasies of an insane man who has dreams of murdering people. His walk, body motion and make up are the bases of modern day zombies.
The first appearance of the popular North American version of zombies was in the 1932 film White Zombie.
The original zombie was more of a
Another thing that has change is the gap between death and zombie. The original zombie was often a long since dead cadaver re-animated, and today's zombie often come from the freshly dead.
The most significant change since they first appeared is the introduction of the
So from re-animated Frankenstein zombie, to the living being turned into zombies and then the development of the zombie horde. That would be your progression in their history.
This WSJ article gives insight into the cinematic evolution of zombies, particularly into infection-spreading creatures.