As a quick disclaimer, this is not legal advice but general thoughts to hopefully guide your analysis. A vesting schedule is a discretionary tool used by Boards to help ensure they are only giving away equity to loyal employees. Therefore, if I were to join a startup tomorrow and negotiate a two-year vesting schedule, vesting monthly, that should result in the same vesting as if I were on a four-year vesting schedule with a vesting commencement date of October 1, 2021. Thus, backdating the vesting on a new grant is permissible. As you've acknowledged, there are other implications to this around his holding period, and if the strike price is different today than it was when the NSO was granted, that could also impact the new grant. Furthermore, if the new ISO grant had an early exercise opportunity, that could result in an 83(b) election needing to be filed. I hope that helps guide the conversation and feel free to contact me directly at firstname.lastname@example.org if I can further assist.