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If you are talking about the flexible/refundable fares the airlines have been pushing more heavily recently, these typically allow same-day flight changes without having to pay fare differences. But if you want to change to fly on a different day, you will pay fare difference. However, there are often cases of fares that use the original ticketing date to measure the advance purchase time when you have taken the outbound flight and are only changing the return flight of a roundtrip booking. This can result in substantial savings over simply purchasing a new ticket on a flight as you can qualify for substantially cheaper fares with lengthier advance purchase requirements as well as cheaper fares that require roundtrip bookings (still prevalent on many international routes) since you still meet the roundtrip booking requirement.