changing username, how?

hi

asp.net 3.5

I've developed a website which is using SqlMembershipProvider as
MembershipProvider. Database is a sql server 2005 database.

Now the client wants users to be able to change username on the website, how
can this be done?

(change because users are using their mobile number as username, and they
want username changed when they have a new mobile number)

any suggestions?
...