Saving every replay on every song you ever play... That doesn't make any sense. Are you going to be watching all of those replays later on? Probably not. Your time would be better spent just PLAYING and manually saving the replays that you think you really did good on and would like to watch at a later time.
Also, remember that your replays will be saved online if you score in the top 40 for a difficulty. So as your playing improves, you won't have to worry about losing replays.