Function sort(s As String) As String
    Dim As Uinteger i, j, n = Len(s)
    Dim As String temp

    For i = 1 To n
        For j = i + 1 To n
            If Asc(Mid(s, i, 1)) > Asc(Mid(s, j, 1)) Then
                temp = Mid(s, i, 1)
                Mid(s, i, 1) = Mid(s, j, 1)
                Mid(s, j, 1) = temp
            End If
        Next j
    Next i

    Return s
End Function

Function rfind(c As String, s As String) As Uinteger
    Return Instr(s, c)
End Function

Function nextHigh2(n As String) As String
    Dim As Uinteger hi = Asc(Right(n, 1))
    Dim As Uinteger i, ni, idx
    Dim As String sr

    For i = Len(n) - 1 To 1 Step -1
        ni = Asc(Mid(n, i, 1))
        If ni < hi Then
            sr = sort(Mid(n, i))
            idx = rfind(Chr(ni), sr) + 1
            Mid(n, i, 1) = Mid(sr, idx, 1)
            Mid(sr, idx, 1) = ""
            Mid(n, i + 1) = sr
            Return n
        End If
        hi = Iif(hi > ni, hi, ni)
    Next i

    Return "0"
End Function

Dim As String tests2(8) = { "0", "9", "12", "21", "12453", _
"738440", "45072010", "95322020", "9589776899767587796600" }
Dim As Double t1 = Timer
For i As Uinteger = 0 To Ubound(tests2)
    Print tests2(i); " => "; nextHigh2(tests2(i))
Next i
Print Chr(10); Timer - t1; "sec."