It is totally dependant on the fare rules, and as I once found out the way the checkin agent interprets them. So on that occasion the commitment of the airline was non existant as they refused to issue me with a boarding pass for a clearly changeable ticket which I had earlier changed using their phone customer service agent. Then they blamed me as I clearly should have known this, but the customer service agent must have been new and wasn't expected to
If you have a full fare ticket and they play silly games you can just get your money back and rebook on another airline. If you have certain status and book a few days in advance most airlines will guarantee you a seat in economy if you pay full fare even if the plane is full. Not that it matters, as they regularly sell seats to punters when the plane is full, but they get more money off you that way.
If you think you might change the return date the best way to do it is to purchase a discounted ticket where the fare rules let you pay a fee, usually around £100/$150, and they put you on the next flight where a seat of that fare type is available. That might not be the day you want, but usually they can get you home within a reasonable period. You usually need to make the change before your ticketed flight departs. If you buy the cheapest cheapest ticket you will probably be unable to do this. If you purchase the ticket through a consolidator the airlines often don't know what the fare rules are, so get the consolidator to put in the notes that you can change the ticket and how much. It's all a joke really, they will enforce rules but if they don't know what they are then they can make it up. If you want to change the outbound flight on a cheap ticket you are generally out of luck.
So that's my experience, others might be able to add/change things.