Welcome to FT!, nichehole

You are correct there will be no change fee but may be a fare difference

However, then it gets tricky

Generally OW international tickets get pricey. A few international markets that is not true but for most it is
In most cases an independent OW will be nearly the cost of a round trip. You need to check your routing for this.

Next issue - the future flight credit for a partial flown trip comes with restrictions. It retains some of the use restrictions of the original trip.

There be little or no difference little difference from changing your trip vs cancel your trip and use the credit to book a new return.

I would suggest making any change with a phone agent, the case of change a partially used ticket can get funky especially if it is a very different route. Probably less of an issue change for essentially the same trip but different dates -- but still a good thing to talk this thru with an agent.

As how much of a "credit" you will have, without detailed access to your ticket, that is impossible to predict. And it may change if you change the return from the original routing / carriers. If the fare classes are the same both ways, it may be a 50/50 split. If different the higher fare class direction will be allocated a greater amount.

One last thing, sure about the "X" fare class, there is usually an award fare class.