How much did the other writers change? Well not the tone but here is the Koepp first draft in one sentence: Ock was created by an accident caused by Norman Osborn. Osborn wants Peter to join him and then they get in fights and Ock keeps showing up and mucking it up and tries to kill Norman.
At the end Norman lives, Ock and Norman go to prison and Spidey and MJ make out on a web from a chopper over the brooklyn bridge after Osborn fell off it, and Harry Osborn watches and mutters "SPIDERRR-MANNN!" and Doc Ock speaks in all caps for the most part (think Arnie in B&R) and there is no bittersweet ending.
So they changed the entire storyline and certainly added what real humanity was there as there was none in Koepp's draft. Why do you think Sam didn't want him back for 2 and outside of having to endure the Smallville writers, went essientially right back to Sargent. There is no middle man for 3 now.
And Koepp has written some good screenplays, if generic. But I give credit to Jurassic Park to the actors, director, SFX and John Williams more than the screenplay (as opposed to say Jaws). Also, The Lost World was a huge disappointment as was War of the Worlds. For that reason, I am lukewarm on him doing this. And the less said about Secret Window the better.