tl;dr: I have a ₤2000-ish ticket, I want to change it to the one below which is at ₤2300. I would expect the change fee to be something like ₤500 (so the difference between them, and change fee?), the change will be moving the date of the outbound to be earlier, and to a different destination. But same cabin, PE/WTP. The inbound First flight however seems to have made any change to my booking super expensive, any change I've tried exceeded both prices by a few hundred.

And I wonder whether it's worth calling them one more time, quoting the price I have from ITA Matrix.

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Hello,

Few weeks ago, I've booked a return flight LHR→BOS for a 1w period. PE out, First back. Some plans have changed and I want to extend the trip and spend a week and a bit on the West Coast first.

Having looked at prices of PE flights to SJC, which were in fact cheaper than the PE flight to BOS in the ticket I had, I kinda hoped this would be a simple and cheap change...

However, I've called BA twice now, and twice I was given a ₤3k (additional!) price for this change, which is 50% more than the price of the original ticket! This seemed to surprise their staff as well and they've done some experiments, but no luck. Second person I spoke to, did eventually notice that the system was forcing them to make the outbound flight Business/CW instead of PE/WTP which of course explains some of this. (First person mentioned a "at least 50 days ahead" rule which applied to my F fare but .... not sure about that given my next disovery?)

Now I wonder, is this one of the mysteries of flight change charges? I've finally figured out now how to do mixed-class open-jaw in ITA Matrix, and even there I get an overall ticket price much closer to what I've expected of ₤2300 (screenshot attached to this post), only a little over the price of the ticket I have and obviously/clearly less than the CHANGE fee they've quoted me...

Given how ITA Matrix has now given me a fare that looks like what I've expected, is it worth calling BA once more and if so, what should I tell/ask them? Or, are they going to tell me that ITA Matrix is perfectly able to produce tickets that cannot actually be issued and should I really just give up on this?

Anyone's expertise on this very much appreciated. Wish I had just made my plans properly from the start... (Or perhaps, that I had resisted the urge of booking First on the inbound!)