Mods, please merge into an existing thread if there is one already...couldn't find one so figured I'd start by asking my question here.

I'm looking at purchasing an AirPass, as I'll be doing quite a bit of commuting between the US and Europe next year. Some travel will be able to be planned in advance and some will be more last-minute. I typically book discounted business fares, although if the price is sky high will book Y and use upgrade instruments.

I spoke with an AirPass representative the other day, and was informed that Instant Upgrade tickets are essentially economy fares that instantly book into business, based on availability. What she wasn't able to answer is which fare bucket these upgrades come out of, especially on international routes such as CLT-FRA, and how often upgrades actually happen. Are these upgrades pulled from C inventory, and if so, are they as difficult to score as a regular SWU or miles+copay? Or, is inventory created when making an Airpass booking that otherwise wouldn't be seen when making a "regular" booking (ie. a seat pulled from I or D inventory)?

If AirPass upgrades are as scarce as general C inventory and you end up sweating out battlefield upgrades like the rest of SWU holders/miles+cash upgraders, then the value to me would be low, especially if booking last minute and ending up on the bottom of the totem pole for time stamps. Can anyone comment on how well the program works for them, specifically on TATL trips, and how the upgrade priority works/how easy it is to actually upgrade on an Instant-Upgrade fare? Paying ~$4500 for a CLT-FRA fare and sitting in Y isn't particularly appealing to me if that is what would would usually happen more often than not.

Thanks for any insight!