I've experienced similar problems twice in the last month. In both cases, my flight had been swapped to a different one and the new flight was ineligible for online upgrade. The second time it happened, I contacted my local ET office and they fixed it for me. I'm not sure what they did, but the flight became eligible for online upgrade and I successfully bid for a seat in business class. So this obviously happens from time to time and it seems ET staff can rectify it for you assuming there are seats available and time available to get it sorted. YMMV.