Do you believe that a man's nature can be so easily be changed as Petruchio was able to change Katherine?

No, I don't think it is easy to change a man's nature and I am not convinced that Petruchio was able to change Katherine. I think that Katherine simply learned to adapt to behavior that was in her best interest. She learns that it is better to use "obedience" as a manipulation tool rather than outright defying Petruchio at every moment. Katherine was not "tamed" more than she became wiser to the power dynamics between men and women.