Upvote:1
One common method airlines use to calculate new fares is to recalculate the entire itinerary (including any part you have already flown) as at the day you make the change, and charge you the difference. The exact method will depend on the rules applicable to the fare classes you booked.
If the new fare is lower you might get a credit or refund, or you might forfeit the balance, depending on the airline policy.
Should you buy a return or two one-way tickets? All I can say is that tickets bought well in advance tend to be cheaper than tickets bought close to departure.
Since you're also getting a return-ticket discount I'd expect that booking a return ticket now and changing it will work out cheaper than buying a one-way ticket now and another later. However, depending on the exact changes you make and when you make them, that is not guaranteed.