Решение, основанное не на «Skim» или «MP», а на% -знаке:
Sub tst()
Dim mystring As String, y As Variant, i As Long
mystring = ThisWorkbook.ActiveSheet.Range("testScore")
y = Split(mystring, " ")
For i = 1 To UBound(y)
If InStr(y(i), "%") > 0 Then
If Val(Split(Split(y(i), " ")(0), "%")(0)) < 70 Then
mystring = Replace(mystring, Split(Split(y(i), " ")(0), "%")(0) & "%", "f")
Else
mystring = Replace(mystring, Split(Split(y(i), " ")(0), "%")(0) & "%", "p")
End If
End If
Next i
MsgBox mystring
End Sub