Definitely agree availability of full redemption ticket is a good indicator but not 100% reflective of mileage upgrade availability. I can recall at least two datapoints that shows it could go either way:
- Attempted Y+ to J upgrade for HKG-YVR around 2016. Saw on CX.com redemption tickets for J were available but MPC hotline agent told me no upgrade space available. Upgrade finally confirmed at check-in counter.
- Recently attempted Y to J upgrade for HKG-HKT. CX.com showed redemption ticket available and sure enough the call agent was able to confirm the mileage upgrade for me.