admin cannot modify a username to one that contains a reserved word

[mousetemple]

Hi all,
i have set the option in registration options to make isReservedName match the entire name, not just part of name,  So a user can register with a name say 'guest_fred'.   However, if i, as admin, want to modify a username to something like 'guest_fred', that is recognized as a reserved name in the modify path.     Is there an option somewhere that i can tweak to allow this, or is this a defect?   

Thanks