As a spin-off of the 'bands who have produced two almost equally great records' thread, how about bands or solo artistes are there who have radically changed style over the years, and in the process produced two or more records which are stylistically very different but both amazing?
You could argue that OK Computer and Kid A (or at least The Bends and Kid A) is an example of this.
Also, for me:
Scott Walker - Scott 4 & The Drift
Wire - Pink Flag & 154
The Beatles - Help! & the white album
XTC - Drums and Wires & Skylarking
So any others? And is this the ultimate testament to a band's greatness?