I have a personal profile for a business as well as a business page. I plan to delete the former, but unfortunately that's the one that has the desired username. If I delete that profile, will the username become available, or will it be lost forever?
This is no longer true. According to this thread (as recently as 2017): https://www.facebook.com/business/help/community/question/?id=10153399860456687 the policy is that a username, once used, is never available again, even if the page that originally used it is deleted.
I am coping exactly the same issue. I proceeded to modify the username on my personal account in order to reused it on a new business page (created into my same personal account), but when I tried to enter that username (that I had released a few seconds ago by myself) the system showed a warning "username not available".
Can someone tell me how to recover my original username, that by the way nobody is using so far?