I was looking at doing a Turkey Week hop to Japan or HK and was noticing some odd upgrade behavior when checking availability online.

Logged on as a Plat, I found no upgrade seats when I did a basic search - but when I changed the search to 'Advanced' and entered 'B' as my fare basis, upgrade seats to NRT and HKG magically appeared. When I changed the search to the lowest fare option, the same flights showed no upgrade availability.

Perhaps I'm just not remembering something - but is upgrade seat availability tied to the fare bucket you choose, or do certain fares (Y, B, M) pull from a different pool?