This is a manageability issue for larger installations... I've hit this twice: once, in my company's system, when a staff member got married and changed her name, i.e. from jdoe to jsmith.
I also hit this with my own username on this system; my username is still 'firstname.lastname@example.org' even though I no longer work at Notiva.
If your underlying data model utilized a user ID for the foreign keys instead of copying the actual username into the field, this would be a trivial matter.