Hello jszei and welcome to the board.

Well an issue with your approach is that the name often will not be OUTLOOK.NK2 but may be the name of the user's logon ID or there may be multiple names in the same folder due to multiple profiles by the same user maybe even on a shared machine.

Please give this a try and see how it works.
 
Code:
Dim $NK2
$NK2=ReadValue('HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders', 'AppData')
If Exist($NK2+'\Microsoft\Outlook\*.nk2')
  SHELL '%COMSPEC% /c REN ' + '"'+$NK2+ '\Microsoft\Outlook\*.nk2'+'"'+' *.old'
EndIf