Is this kind of what you are after?
Sub teset3()
    Dim a, i As Long
    With Range("a18", Range("a" & Rows.Count).End(xlUp))
        a = .Value
        For i = 1 To UBound(a, 1)
            If a(i, 1) <> "" Then
                If a(i, 1) Like "*/*" Then
                    a(i, 1) = Trim$(Split(a(i, 1), "/")(0)) & "/" & _
                    Format$(Val(Split(a(i, 1), "/")(1)) + 1, "000")
                Else
                    a(i, 1) = Left(a(i, 1), 10)
                End If
            End If
        Next
        .Value = a
    End With
End Sub