SQL statement assistance - how to update field with same value from another field in same table

Hello,

We are on RHEL 6.5, MySQL 5.1.7 & PHP 5.3.3 and running moodle 2.6.4+

Table: mdl_user;

We would like to populate the "idnumber" field with the same value in the "username" field globally.

How could we achieve this through an SQL statement?

I don't think we can use the cli syncronization script (which accesses our external database) for legacy ones already in moodle but we can use that script moving forward. Come to think of it, what happens if we run the script again? 

Thank you everyone.