Here's my experience - On Nov 7th 2022 I had booked SIN-JFK in J (advantage rate) for travel in Aug '23. Then earlier this year I needed to change my route to EWR-SIN for Nov 22nd, 2023. Then, a few days ago I had to change my ticket once again, this time for travel in Aug 2024. I had no issue having the dates/route changed and because it was an advantage ticket, I was also not charged a fee.

I found the chat agents to be more knowledgeable and helpful with the ticket changes vs. the phone agents. Two chat agents also confirmed to me that while for revenue tickets the new travel date needs to be max 1 year from original ticket issuance date, for award tickets it's 1 year from last ticket issuance date.

Of course, as always, YMMV