Hi, I mentioned this in a previous post, but I booked a revenue ticket for JFK-IST-BKK-IST-JFK. So far, all four legs of the ticket have had involuntary changes (lol) and I'm planning to take advantage of that to change the dates of my flights.

My questions are mostly concerned with the fare class. How does TK determine the new fare class after an involuntary change?
My initial tickets were in class L and K (I paid cash to upgrade the return to business); when I attempted changing the ticket online, it showed (right before pressing the confirm button) that the new ticket would be in class U and Z. If I understand it correctly, U is worse than L since it accrues fewer miles.

Now, I've heard that if I wait to confirm the tickets late, there's a chance that the cheap fare classes will have sold out and I will get a higher fare class (maybe even Y or B so I can use SAUA?). Can anyone confirm if that's correct or not with TK? If it's true, what are the drawbacks of waiting to confirm the schedule change till very late, like a week before my departure? Any risks I should be aware of?

Sorry, that's a lot of questions. Any insight is welcome!