I just had an exchange with someone at reservations and have genuinely no idea what is going on. Am hoping those with more experience can shed some light.

Had to call to change a YOW-YYZ-FRA-EDI flight from mid Dec to mid Jan. This is a latitude fare, the return leg of a round trip which had eups applied, which means for some reason AC splits it into a multi-city and it's impossible to explore or price out changes online. I called and asked about a flight on Jan 15. Was hundreds of dollars more for some reason. Okay, asked about a couple of other flights on different days. All were a few hundred more. That seemed weird. It quickly became clear that it's impossible to explore pricing when I have to ask about each flight individually and then wait a minute or two while she gets the price. I had the idea that if I just change the flight to a date where the routing is YOW-YYZ-EDI, all on AC metal, dropping that connection through FRA, it might fix that multi-city thing and allow me to search for options online. So I ask to do that, picking May 1. Again it was hundreds more but I didn't worry about it as that was a temporary flight date only.

I'm puzzled though and ask her why this new flight is so much. She tells me "your original base fare was 625 GBP and the new base fare is 917 GBP".

After getting off the phone, I look up the old, original booking. Base fare is 1061 CDN (which = 625 GBP) and there are 407.50 CDN of carrier surcharges, so air transportation charges were $1465.50 CDN, about 865 GBP. The new receipt for the new, changed booking says 'air transportation charges' (not base fare) are 917 GBP. There is no mention of what the base fare is, except if you subtract the carrier surcharges listed from the air transportation charges, it's around 660 GBP. That suggests the base fare was about $60 more, not almost $500 more.

The new receipt also seems to show I owe/ just paid 402 GBP for this changed flight, but I have no idea where that number comes from. (She told me what I was being charged but I couldn't make out what she was saying and I didn't care what it was exactly because I figured it was a temporary booking anyway.)

Can anyone decipher this receipt for me or give me a sense of what has happened? I have often had currency conversion problems with AC so their receipts are actually not accurate, in that they will show something like 675 Euro when it's actually 675 Canadian. Has something like that happened? Or if ATC are 917 GBP, and I already paid 865 GBP for ATC, shouldn't the additional charge be modest? Where does GBP 402.00A come from? And the 317 CDN NON-REF under endorsements, what does that mean?

Any help very much appreciated.