Dorian Gray pretty much was the typical dandy from the start of the novel. He was simply not yet awoken to the sins of pleasure that were put forward to him by Lord Henry.
Within this paradigm there are two major changes: First, his supposed "falling in love" with the poor actress Sybil Vane. Of course, this was merely caprice, but her suicide as a result of his abandonment made him more aware of how influential he was with people.
"I knew that I had come face to face with someone whose mere personality was so fascinating that, if I allowed it to do so, it would absorb my whole nature, my whole soul, my very art itself." (Basil, about Dorian, ch1)
So from that moment on, he said to have had a form of rebirth where he want to experience the intensities of life through pleasure.
"There were moments when he looked on evil simply as a mode through which he could realise his conception of the beautiful." (on Dorian, Ch 11)
Secondly, came his debauchery period. He would enter the slum district of East London and participate in all forms of deprived activities, attended opium dens, and it is understood between the lines (and quite on purpose by Wilde) that he was practicing homosexual activities as well. This is an interesting fact because by this time in Victorian England, this was a severely punishable crime. As this period got worse and worse, he would remain young, and his picture would reflect the malice and nastiness of his soul.
"You look exactly the same wonderful boy who, day after day, used to come down to my studio to sit for his picture. But you were simple, natural, and affectionate then. You were the most unspoiled creature in the whole world. Now, I don't know what had come over you. You talk as if you had no heart, no pity in you. It is all Harry's influence, I see that." (Basil, Ch 9)
The most important change came when he began to make every man in London lose his reputation when seen with him. Society began to shun him, and he even drove another man to suicide. When confronted, Dorian went to the ultimate moment of insanity by killing the very man who loved him the most, Basil. Basically, Dorian went from friend to foe, and from sane to insane. He also turned into a heart breaker, and cold blooded murderer. In the end Dorian killed himself, making the picture finally turn back to its normal self while in the floor laid a dead old horrible man.
"Nowadays most people die of a sort of creeping common sense, and discover when it is too late that the only things one never regrets are one's mistakes."