I understand that in case no advance seat reservation has been purchased in Eco class, during online check-in (starting from T - 24 hr) a random seat will be assigned by the system, and that it can only be changed by paying for it. Correct?

On the TK website there is a distinction between T-24 hr and T-6 hr. Is there any time window during online check-in when seats can be changed free of charge? Does *A Gold status make any difference here? At the airport, would the check-in agents or gate agents change the seat free of charge?