I had my original ticket CPH-SIN for May 2020, then got reschudled for Feb 2021, then again for June 2021 (at the second attempt, cause the first agent told me it was beyond the expiry date), and yesterday I managed to reschedule it again for Feb 2022 - at the LOT ticket desk in Warsaw- the agent did it with few clicks and my new ticket was already in my email ;-)