Replace based on a value in a cell

I want to have a find and replace function that finds and replaces values on a sheet based a cell containing the find value and a cell containing the replace value.

At the moment i have

Sub update_month()
'
' update_month Macro
' Macro recorded 27/03/2007 by sbarber
'

'
    Sheets("P&L - Sales").Select
    ActiveCell.Offset(0, -4).Columns("A:A").EntireColumn.Select
    ActiveCell.Offset(-25, -4).Range("A1").Activate
    Selection.Replace What:="C[2]", Replacement:="C[3]", LookAt:=xlPart, _
        SearchOrder:=xlByRows, MatchCase:=False, SearchFormat:=False, _
        ReplaceFormat:=False
End Sub

which will find C[2] and replace it with C[3] but i want both of the values to be entered into two cells on the work sheet rather than having to amend the code each time?