Gos-C

Try this macro - It does not verify Letters & Number are placed in the correct order - This can be added if required.

Copy this macro
Goto Excel
Right Click on Sheet Name Tab > select View Code
Past macro into the Worksheet Module displayed

Change Column number to suit (shown In red in this posting)
Return to Excel and test


Private Sub Worksheet_Change(ByVal Target As Range)
   Dim Rng As Range
   
   For Each Rng In Target
      If Rng.Column = 1 Then
         Application.EnableEvents = False
         Select Case Len(Rng.Value)
         Case 6
            Rng.Value = UCase(Left(Rng.Value, 3) & " " & Right(Rng.Value, 3))
         Case 7
            Rng.Value = UCase(Rng.Value)
         End Select
      End If
   Next Rng
   Application.EnableEvents = True
End Sub