I-9 does not have to be for new hire always. It's a document employer always need to "keep current" depending on employer's status. For that reason, for PR or citizens employer' needs to do it only onetime and that's the time of hiring. However, for alien workers (execpt PR), employers always need to replace I-9s if required. There is a section (section 3) for eligibility reverification, but that can be done only one time as there is not much space for re-verifications multiple times. In that case, employer always need to obtain new I-9 from employer. For example, my employers used to ask for new I-9 always (ignoring section 3). It's employer's responsibily, but it's always better to keep employer informed about statue change. Once you give your employer your new I-9 (with a copy GC stamp/card), that would be the last time you have to fill-up I-9 with your current employer.