Unfortunately, there's no way to know which usernames changed without scraping all of them.
I don't think so, Loyce. I don't have skills to do this but I imagine that you only have to scrap username of active accounts. When one username changed, that owner most likely log in that account to send request to admin/ global moderators and if request accepted, admin/global moderator will do something that likely trigger that account from inactive to active.
I know you don't have data on active/inactive status of all users so I know this task is challenging for you.
