Always grateful for your reading and comment, @hlezama. When Rimbaud speaks of "changing life," he is obviously referring to life as existence, to put it in a more philosophical word.
The precision you make about Rimbaud's phrase is correct; indeed, the verbal and semantic play is that it "dispersonalizes" the expression: not "I am another," but "I is another". One of the contributions of his vision is precisely to establish that distance between the
"I" empirical and the "I" poetic.
Greetings.