I'm in a similar situation. I have a T class WTP return to BKK booked in October. For both legs the cash upgrade is available (each at around £1000 - no thanks, but I'd be tempted at £299) but both miles upgrades are unavailable.

I was hoping to use miles to upgrade at least the outbound leg to CW but I guess BKK is a busy route for redemptions. I'll just keep checking.