Manually Changing Username In Membership?

I am able to change a username by directly accessing the asp.net membership user tables. However, the old username is preserved in a new row and assigned a new UserID automatically by asp.net. How do I stop that from happening?

EDIT: only in the users table and roles table, not the membership table.

var mUser = dc.aspnet_Users
.Where(u => u.UserId == (Guid)user.ProviderUserKey)
.Select(u => u).SingleOrDefault();
mUser.UserName = newName;
mUser.LoweredUserName = newName.ToLower();
try
{
dc.SubmitChanges();
}
catch
{}