Well, the official word from Cathay/CMB is
"If your reissuing the ticket you need to pay the fare difference to the current AONE4 fare which is LKR980,000. Also you need to pay the reissue fee USD125 plus any taxes."

The base AONE3 fare last year was LKR769,400, so the difference is about USD2000, using xe.com/ucc for mid-market conversion.

Not a sparkling bargain. I was under the impression that the fare difference would continue to be based on the fare table in existence when I first booked. Live and learn.

BTW the spreadsheet, which says it was updated just a few days ago, seems to have fallen victim to recent increases. It still lists CMB AONE3/4 fares at USD7079/7554. EF gives USD8146/9021. The EF AONE4 price matches Dulani's number above to within pennies, using xe.com conversion. That's an incredible increase - when did that come about, and did it affect other popular ports?