As stated earlier, that needs a macro, not a formula.
--
David Biddulph

As has been written, you cannot do that with a formula.  A formula can only
return a result to the cell in which it resides.  It cannot change another
cell, nor can it change itself, yet persist within the cell.

Take another look at Gary's student macro.

Or, if you want this automatically, use an event macro.

To enter this, right click on the sheet tab and select View Code from the
dropdown menu.  Then paste the code below into the window that opens.

=========================
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
Dim a As Range
Dim c As Range
'set up to change entries in Column A only
Set a = Range("A:A")
If Not Intersect(Target, a) Is Nothing Then
   For Each c In Target
       Select Case c
           Case Is = 400
               c.Value = "James"
           Case Is = 401
               c.Value = "John"
           Case Is = 402
               c.Value = "Kevin"
       End Select
   Next c
End If
End Sub
=================================
--ron

If you want to keep it simple, you could always copy the formula col at the
end of the day, then overwrite the source col with a paste special as values