Marlow's change is more obvious, since he is relating the story and explaining how his views became altered. He started as a hard-working, young man, without existential worries or fear of mankind's inner brutality. Each event serves to wear away some of that certainty until he is consumed by pessimism and indecision, scared of the darkness he observes in others and terrified that he himself may be capable of such things. His change is overt and specific.
Kurtz's change is explained, but not present as an event; when the reader finally sees Kurtz, he is the final throes of his jungle-induced insanity. He has, by the sheer power of his personality, taken control of the ivory trade and cowed the native peoples into submission with brutality and eloquence, but in his quest for power he has gone too far from his civilized morality. When The Intended, his lover, speaks of him, she describes a man very different from the Kurtz that Marlow met:
"'...Who was not his friend who had heard him speak once?' she was saying. 'He drew men towards him by what was best in them.' She looked at me with intensity. 'It is the gift of the great...'"
(Conrad, Heart of Darkness, gutenberg.org)
And yet, many of her descriptions work for the insane and self-glorifying Kurtz who died, in his own mind, alone in an uncaring world. It is more likely that Kurtz always had the potential for his insanity and cruelty, but kept it locked inside as long as he had civilization as an example. The uninhibited jungle allowed him free reign, and that freedom destroyed his mind and body.