If you're referring to the rules about cancelling before flight departure, then it will vary from ticket to ticket.

In general on BA, changeable but non-refundable tickets must actually be changed prior to the departure of the flight, otherwise you forfeit the ticket. You can't just call them a few months (or even days) later and select a new flight. It's a bit annoying if you're sick on the day of the flight and don't know when you'll get better, or indeed if you'll get better in time to make the trip worthwhile at all because there's often a significant fee each time you change, but it is what it is. I'm not quite sure if that means you can call literally five minutes before departure, having missed your last chance to check in, and change, but I believe on some flights that is the case. I've had to change BA tickets ex-USA and I think they work like that as a general rule.