Visio: Employees And Managers

Ok, I have the following table

CREATE TABLE employees (
employee_number char(6) NOT NULL
, known_as char(20)
, surname char(20)
, job_title char(20)
, manager_number char(6) NULL
, unique_identifier char(6) NOT NULL PRIMARY KEY
)
--unique_identifier is in the format 000123456789

Now I have a conundrum when trying to create organization charts in Visio, so I figured I'd try reproduce the format that a bunch of walkthroughs suggest, which is with the first column being an int identity(1,1) column as the employeeID with the managerID being an int column also, referencing the employeeID.
Hope I've not lost you just yet ;)
So here's what I figured - create a new table with the new integer columns, slap my current data into it and then update the managerID as necessary...

Except I can't work out the update statement for this!

CREATE TABLE gvVisioTest (
employeeID int identity(1,1)
, employee_number char(6)
, job_title char(40)
, department_reference char(10)
, managerID int
, manager_number char(6)
)
GO

INSERT INTO gvVisioTest(employee_number, job_title, department_reference, manager_number)
SELECT employee_number, job_title, department_reference, manager_number
FROM employees
GO

--Update managerID with relevant employeeID
GO

DROP TABLE gvVisioTest

Any ideas?

Oh and this is legacy so the design is flawed, modified over time (the manager field is a bodge put in 5 years ago), so yeah unfortunately I have to work with what I've got :(