Unless I'm mistaken, in Fleming's universe designations like "M," "Q," and "007" are titles, not appellations. So Nigel Codswollop can rise through the ranks of MI:6 and, when he needs a disguised identity, can be "007" as far as his bosses are concerned, and can assume the name "James Bond" when he's working. When Codswollop dies or retires or moves on, another spy - say, Emily Nottinghamshireboltonupontyne - can take on the designation "007" and take on whatever name he or she wants.