Вот код, который это сделает (проверено на примере в моем Excel 2003):
Sub t()
Dim rCells As Range, c As Range
Dim arr As Variant, i As Integer
'Define the range to apply the code
Set rCells = Range("A1")
For Each c In rCells
'check if the cell desserves to be changed (could be adapted though to another check)
If Left(c.Value, 4) = "pos=" Then
'split all the values after the "pos=" into an array
arr = Split(Mid(c.Value, 5, Len(c.Value)), ";")
'add +2 to every value of the array (we convert the value to be sure, probably unneeded)
For i = 0 To UBound(arr)
arr(i) = CLng(arr(i)) + 2
Next i
'set back the value to the worksheet
c.Value = "pos=" & Join(arr, ";")
End If
Next c
End Sub
Обратите внимание, что я не добавил часть проверки ошибок, если ваши значения не отформатированы.