“I want to change my return flight, which will cost me a $200 change fee. I found a cheaper one-way ticket. Is there any reason I can’t just buy a new ticket? “

You can certainly purchase and use a new one-way ticket so long as you observe one of two simple guidelines. 1) Do not cancel the original return flight until after the outbound has been flown. To do so beforehand would convert your ticket to a one-way outbound ticket at the one-way price plus the $200 change fee for doing so. 2) If the new one-way ticket is on a different airline from the original ticket it is possible to purchase it at any time and not cancel the original return flight. There are no financial repercussions to doing so, though there may be ethical ones since someone else could have used your unwanted space. In most cases, a partially used ticket will have no value for future travel.