If your arrival flight changes after you arrange an onward ticket, check the new arrival date against the onward departure date before you travel. You do not always need to replace the onward reservation, but you do need an itinerary that still makes sense. A departure flight dated before your new arrival is an obvious problem; a departure that leaves almost no time in the country may be just as impractical.
Flight changes are annoying enough on their own. The good news is that reviewing the onward side usually takes only a few minutes when you approach it in the right order.
Start with the new arrival, not the old reservation
Suppose you booked a flight to Bangkok arriving on 5 January. You expected to spend twelve nights in Thailand and arranged onward travel for 17 January. Then your inbound airline moves you to a flight arriving on 9 January.