The change about Nemo's origin must have been done before finishing "Twenty Thousand Leagues Under the Seas", because already in that there were expressions of

The change about Nemo's origin must have been done before finishing "Twenty Thousand Leagues Under the Seas", because already in that there were expressions of his discontent with the British Empire, above any others, I think towards the end of the novel, when he attacked some British war ship (an attack that was used to convince the more reluctant of the prisoners that they must escape from the submarine). That novel has been published in episodes scattered over more than a year, so it would not be unusual if towards its end Verne had a different vision of the story line than when he had started writing it, one year before.