It is correct that SQ phone agents are only going to follow ticket rules. We don't know what these are specifically so we can't really comment. Inventory in your specific booking class may open up, or it may not. You may have change fees, and the fare rules may require replacing using current or historical fares. We just don't know unless you post your precise fare basis.

You can either call back from time to time to enquire about the possibility of changing (keeping in mind what I wrote above), or you can simply ask on the day of travel when you get to the airport.

I agree that I can think of worse places to cool my heels than Singapore.