ASP.NET 2 Membership does not allow update user name

[Rich]

If user name "user" added to aspnet membership db, it will not allow
update user name. eg change "user" to "User" (different case).

Tried workaround by deleting "user" from db and then adding "User" to
db. Membership saved it as "user" despite being instructed to save
"User".

Is this a bug in the membership?

Any feedback appreciated, thanks.

Rich