Phoned EVA's SF office, and a nice lady with a marked Chinese accent but excellent diction changed my return flight to the date I wanted. Some trade-offs, mostly good. The original return Bangkok-Taiwan leg did not offer the "elite class" which I had paid for, but the new flight does. The new flight leave late afternoon instead of mid-morning, which is nice. It gets into SFO at 8:30 pm which is later than I wanted, but workable. The change gives me another weekend in Thailand.