If I use the EntityData command at any entity to change the type of the entity, the entity doesn't visually change.
Steps to reproduce:
/summon Sheep /entitydata @e[type=Sheep] {id:"Zombie"}
More in-depth:
The entity class doesn't change with the entity data.
Possible fix:
Re-build the entity (use the readNBT function) using the changed NBT to correct the entity type.