Here are some of the changes...
1) The first character you meet in the book is Williams. 2) Bolo is Turkish. 3) O'Harra was a mercenary who used to work for a Mexican drug lord. 4) The Shaolin Monk who trained Lee, also trained Han and implied that he was the best he ever saw. 5) Mr. Han is described as a short man with a muscular build, short black hair, and glasses. 6) Lee's sister Su Lin was 20 years old when she died. She had been sick and bed ridden and was on the mainland to recuperate. On the day she finally felt well enough to leave her home and go outside, she was attacked by O'Harra and his squad. 7) Roper was Williams' commanding officer when they were in Vietnam. 8) It's implied that O'Harra raped Su Lin's body after she had committed suicide. 9) After watching the matches, Han believes that Roper is more skilled than Williams, but doesn't have his speed or strength. 10) Lee kills O'Harra with a knife instead of jump stomping him like in the movie. 11) The fight between Williams and Han was only a paragraph long and didn't end with Williams being bashed to death with Han's iron hand. Instead Han knocks Williams out and then has him taken away to be interrogated/tortured by Bolo. (Williams eventually dies, but only after they had bled him out using strips of barbed wire and then dumped him into the ocean to drown.) 12) Prostitutes with dark skin didn't last long on Han's island because "everyone wanted to sample dark meat." 13) Han sent 6 men to defeat Williams before they fought, and all six were knocked out cold. 14) Before he saw Williams being dropped into the water, Roper wanted to join Han's gang. 15) It's implied that Han's missing hand was lost during a drug war with a Lord from Afghanistan. 16) The man Lee tricks into getting on the small boat was not Parsons but a large Korean Tae Kwon Do master. 17) Up until his fight with Lee, Han had killed every man he had been in an actual fight with. Usually he maimed them first. 18) Lee kills Han by kicking his knife hand into his own chest. 19) Williams never picks any women to sleep with while on the island. 20) Although Lee does manage to eventually kill Han, the book makes it clear that Han was still the better fighter. And that Lee's win came in a final act of desperation. And the fight occurs in Han's private museum and not the hall of mirrors like in the movie.21) Roper doesn't beat Bolo to death, instead he stabs him through the heart with a katana. 22) Williams is from the Watts section of Las Angeles. 23) The organization that recruits Lee is called F.A.D.E. 24) There were children living on Han's Island.