As an NWA Gold I've successfully used the AS upgrades at least 20 times in the last 4 years on the following routes: SEA/PHX; PHX/SEA; SFO/SEA; SEA/SFO; SEA/ANC; ANC/SEA.
The business about the Y-26 fare means that if you buy full fare coach you can upgrade at the time of purchase. Otherwise you can upgrade 24 hours in advance of your flight. AS is the class of the industry, especially in 1class.