Little bit of a plagiarist, eh? Shakespeare and the history of King Lear

The main reason I decided to read King Lear (besides how obviously famous it is) is that in my Medieval British Literature class I actually read the source that Shakespeare drew from when writing this play. Geoffrey of Monmouth, as the Necessary Shakespeare remarks, was a "Welshman in close contact with Celtic legend." He wrote between 1100-1154, and included a history of Lear, or Leir, in his "A History of the Kings of Britain." (This source is cited in the Appendix under "Sources" found on page A-30, in case anyone's interested.) When I read this selection from Geoffrey's "History," I immediately thought that it would be a fascinating experiment to see how close Shakespeare remained to the original source, in characters, plot, and theme. Also, I will analyze the two texts in terms of when each was written, and try to decide what each culture demanded of its literature. Here is a short snippet of Geoffrey's history:

After this unhappy fate of Bladud, Leir, his son was advanced to the throne, and nobly governed his country sixty years. He built, upon the river Sore a city, called in the British tongue Kaerleir, in the Saxon, Leircestre. He was without male issue, but had three daughters, whose names were Gonorilla, Regau, and Cordeilla, of whom he was dotingly fond, but especially of the youngest, Cordeilla. When he began to grow old, he had thoughts of dividing his kingdom among them, and of bestowing them on such husbands as were fit to be advanced to the government with them. But to make trial who was worthy to have the best part of his kingdom, he went to each of them to ask which of them loved him most.

You can find Geoffrey's full text here (starting at section 11). You can see that some of the names have changed slightly, some only in spelling, but that the basic plot remains the same. As I read Shakespeare's version, I'll report if I'm able to find any glaring differences, and then ask why he would make those changes.