Hi, I'm in a bit of a pickle, any help much appreciated so I don't lose my ticket.

I'm in the middle of my very long ANA RTW that originated in the US, started Dec 9 2022 with a big break where I went back home from BCN intending to continue using my own flight to BCN, and fly the following remaining legs:

BCN-MUC 25 Aug LH1809
MUC-IST 25 Aug TK1634
IST-BKK 28 Aug TK58
SIN-NRT (Mid Sep) NH 842
NRT-ICN (Late Sep) OZ 177
ICN-SEA (Oct) OZ 272

I need to push the entire trip forward at least a month from today (a little flexible, but strictly speaking the BCN-FRA has to happen any day starting Sep 16, not before), and I am extremely flexible with any dates and classes from then on (they are all in J now except ICN-SEA, but will fly Y on all to not lose this ticket). The BCN-FRA segment starts in 7 days so I have 3 days to make the rebooking.

When I called ANA yesterday, we started the rebooking and for pushing dates a little like this, I just call without checking availability since I've never had trouble, and I just go down to Y if there's no J availability. Never been an issue since I'm very flexible. This time however, the IST-BKK leg has zero availability for the next year (in J or Y), which I find surprising. This is preventing me from changing the dates of my ticket. Any one have any ideas? They seem to be strict on the city pairing, and the agent said sorry there's nothing we can do to change the ticket now.