Well, create one column in the Main List, where values are changes say "OLDVALUES". Set that field using the properties.ListItem["OLDVALUES"]=Value1+";" +Value2+";" +Value3+";"; in ItemUpdating event(take Value1, Values2 and Value3 using the properties.ListItem["Value1"] and son on).
Now in Item Updating, use like string oldValue = properties.ListItem["OLDVALUES"].ToString(); and slip in array and then you can set global variables and access them in your code. Remember its a SandBox solution approach for event receivers, not for farm solution.
