[Karsh]

Можно использовать тип TStrings:

Код:

procedure gg;
var
  List: TStrings;
  i: Integer;
begin
  List:= TStringList.Create;
  try
    List.LoadFromFile('file.txt');
    for i:= 0 to List.Count - 1 do
    begin
      if List.Strings[i] = '#3' then
      begin
        List.Strings[i]:= '&3';
        break;
      end;
    end;
    List.SaveToFile('file.txt');
  finally
    List.Free;
  end;
end;