I disagree. I was in the exact scenario last year whereby I had a PE ticket in a booking class which was ineligible for an upgrade.

I arrived at the check in desk well in advance and asked if I could upgrade with miles. Check-in agent said we could first pay a small fee to upgrade the fare class itself to an eligible PE fare class, and then pay the corresponding difference in Asia Miles to upgrade to Business.

She was very helpful in spending half an hour to contact back office/MPC to process this - but maybe I was just lucky that time.