It's not that unusual for DL to hold back seats from paid upgrades even on very empty flights. You can always attempt the Modify Flight upfare route and see what it would cost to simply upfare to D1. It's not always the case that the upgrade offers are cheaper than upfare pricing. Also, upgrades are non-changeable and non-transferable should you need to make a voluntary flight change and you will lose the value of what you paid for it. Going the upfare route will preserve the value of your purchase should you need to make a change. There may also be issues with having partner metal on itin inhibiting upgrade offers (Delta does not fly to BKK). Upgrade offers are only provided for Delta operated flights. If you have some Skymiles saved up, you also might want to inquire about Mileage Upgrade Award availability on DL flight. You can potentially waitlist for them if there is no current inventory availability.