Well, by the time I booked it, the "fare closest to my requirements" had gone up again to $1870+ and the actual fare displayed on the itinerary was $500 - so I bought it.

No problem. The e-ticket receipt gives the same low amount.

Then tried to upgrade the itinerary, but the system was "not able to determine the status" of my upgrade request

Looked at the receipt: the fare basis for one of the legs has changed. Instead of T------- in the bottom of the window, it's now 14T-------/WEB. Aaaargh!

So called in by phone and, guess what, was able to upgrade this leg immediately using an HK49.

I've never seen a /WEB designator on a receipt before, but evidently this isn't the same as a fare ending in I as far as upgrades are concerned.