Heres rough example that worked for me. If all of your part numbers have the same format then you could look into formatting strings. Might be cleaner working with it that way.
On Error Resume Next
Dim oDDoc As DrawingDocument = ThisDoc.Document
Dim oPartsList As PartsList = oDDoc.ActiveSheet.PartsLists(1)
Dim partnumber As String = "12141-1123-95-MASTER"
For Each oRow As PartsListRow In oPartsList.PartsListRows
Dim ItemDigit() As String = oRow.Item(3).Value.Split("-")
oRow.Item(1).Value = ItemDigit(ItemDigit.Count - 2) & "-" & ItemDigit(ItemDigit.Count - 1)
oRow.Item(3).Value = oRow.Item(3).Value.Split(ItemDigit(ItemDigit.Count - 2) & "-" & ItemDigit(ItemDigit.Count - 1))(0)
Next