If I book a return business cash fare for LHR-AUH-BKK and I want to upgrade the LHR-AUH and AUH-LHR legs from business to first, the Etihad website seems to say that this is a simple process of paying a fixed 38,500 points for each leg.

Surely I am missing something here ? Otherwise everyone wanting first would simply check first availability, book business and instantly upgrade, even if 9 months in advance.

What am I missing please ?