I confirm the issue where a guest seat in business class is available on the website but the agent is claiming there is no availability for upgrade.
Interesting enough that I asked to clarify which booking classes are needed for a new award booking in business class and for an upgrade to business and both require "I" inventory.
So the issue is probably point of origin, married segments and/ or something else.
Too bad the agents have no idea what the issue is and that both us and EY agents have to waste time looking into these matters.
This is on top of a requirement to wait around a day until married segments are unmarried in case only a part of a through itineary needs to be upgraded. A lot of room for improvement here to both save time (and money for EY) and leave less customers disappointed.