If they are open-dated and the ticket is on paper then you're fine - just throw them away; I've done this more than once. If it's an e-ticket then there's a risk that you may be called on it, but if so then you'll just have to plead that you thought it was OK and offer to do the reissue. But I can't see logically why they might be likely to complain, so I'd take the risk.