very interesting data point:

flying BOD-IST-PTY next week. i've been carefully checking inventory everyday hoping to use my voucher for a last minute upgrade, especially on IST-PTY segment. No J at all, whether on whole itinerary or on each individual segment. I even bought 2 fully refundable biz tix for my parents hoping to save those seats and cancel them last minute so there can be one for me.

i contacted TK on whatsapp to ask if i could use my voucher to upgrade "last minute" my 2nd flight at check in (which is roughly 40 hours before that 2nd flight) or if I needed to wait T-24 for that 2nd flight.

Agent did not reply to my question but to my surprise he said that IST-PTY flight is available to upgrade with voucher right now. no J avail, no voucher upgrade option on website, no upgrade avail even with miles.

I called and they indeed processed my upgrade. at first agent said there was no upgrade available (he checked for both flights) but when i asked him to check the 2nd leg only, it was available. I don't know how that could be possible but there you go!!!