re,
à tester,
Sub test()
t1 = Range("c2")
For i = 1 To Len(Range("c2"))
c = Mid(t1, i, 1)
Select Case Asc(c)
Case 65, 69, 73, 79, 85, 89: t2 = t2 & Chr(Asc(c) + 1)
Case 88: t2 = t2 & "A"
Case 89: t2 = t2 & "B"
Case 90: t2 = t2 & "C"
Case Else: t2 = t2 & Chr(Asc(c) + 3)
End Select
Next
Range("c4") = t2
End Sub