Try pasting this in a new module and hitting F5:

Private Sub SwitchSign()
   Dim iLastRow As Integer
   Dim strAddress As String

   ActiveCell.SpecialCells(xlLastCell).Select
   strAddress = ActiveCell.Address

   Do Until IsNumeric(strAddress) = True And Left(strAddress, 1) <>
"$"
       strAddress = Mid(strAddress, 2)
   Loop

   iLastRow = CInt(strAddress)

   Cells(1, 12).Select

For iCounter = 1 To iLastRow - 1
    If ActiveCell.Offset(iCounter, 0).Value = "abc" Then
          ActiveCell.Offset(iCounter, -9).Value = (-1) *
ActiveCell.Offset(iCounter, -9).Value
    End If
Next iCounter
End Sub

Hope this helps, let me know if you have any problems!
Chris

Middling length but it doesn't destry formulas

Sub test()
Dim myrange As Range
LastRow = Cells(Cells.Rows.Count, "L").End(xlUp).Row
Set myrange = Range("L2:L" & LastRow)
For Each c In myrange
   If c.Value = "abc" Then
       If Not c.Offset(0, -9).HasFormula Then
           c.Offset(0, -9).Value = c.Offset(0, -9).Value * (-1)
       End If
   End If
Next
End Sub

Mike