fttc

I have a repricing question. Let's say I have a round trip ticket with the outbound and return each has a $500 fare. I want to change the return now to a different time with say $400 fare showing up. But let's say the fare class bought on the outbound is no longer available and the next fare up costs $600. So canceling he entire ticket and use the credit on a new ticket is out of question.

Now if I just change the return alone, is it going to be repriced as an entire new ticket (meaning higher price on the outbound and lower price on the return) or only on the one way segments that's changed? This is a hypothetical question, but I like to know what to expect. I would guess it should just be repriced on the one way that's changed, but I'm wondering if the agents/website will stick to that though. Thanks.