Not sure I understand the question. A reservation on hold will just disappear, if you don't pay for the ticket, or you can cancel it yourself on line. If you wanted to make a change, why wouldn't you just start a new reservation?

I made a dummy "held" booking and then went back to it. If you look at it in your itineraries, choose "view/modify" instead of the purchase option, and it allows you to change it. There is no change fee for this; you can do what you want as you haven't purchased the ticket yet.