It needs to operate like the companies house register, as does DBS etc.
One number - one entity. You can change all the details you like after that but never erase the record of them and never create a new one. To, er, 'borrow' a phrase, it should be a contemporaneous record.
Once a person is registered, that's their number forever regardless of what they do. And the sex should be that on their birth certificate at first reg.