Here are the seven suttas having “might not be”. Some use “I”. Some use “it”.
Personally, the “it” makes more sense to me. I have always wondered about the utility of the advice “I might not be” given that a non-returner would just know that they will not be. However, “it might not be” is quite powerful advice for those on the fence between non-return and perfection. It is powerful because the “mine” sneaks into consciousness in exactly the way one bound for non-return would experience it–as subtle warning about the conceit “I am” that manifests as “perfection (i.e., it) might not be mine”.