I think it's a misprint and "it" should be "if". The writer is willing to accept unpleasantness on the condition ("if I could only have") that they experience change and excitement.

It does not make sense to me. At the very least, additional punctuation is needed. Perhaps what the author meant was this: "I was not unprepared for (it = 'jagged rocks and treacherous shoals') -- it I could only have. Change -- change and the excitement of (the) unforeseen." In my interpretation, "it" is the direct object of "have", and "change" is the start of a new thought.