I often run into this problem on intercontinental flights. I have to book a ticket and I am not sure about the return date, which might be after 2-6 months. So I set the return date based on what I know at the moment, already knowing that there might be some changes.
Then I have to change my return trip and usually the change fee is a good % of the original price of the ticket, and these fees have been rising. Example, ticket cost $1000, change fee is $300. This has happened more than once. Then I have a choice of making the change and pay the fee, buy a one-way, or simply not use the return ticket and buy a new return ticket. Is there some formula, or strategy to minimize monetary loss and deal with uncertainties? Unfortunately one-way tickets are often not competitive to the price of a return ticket.