AAirpass' instant upgrade fares book you directly into the upgraded cabin. There is no waiting game. However, these fares are not offered on every route. I don't see them on CLT-FRA and a quick check of a few INTL routes from DFW comes up empty too. I would NOT assume they offer them on your route or, if they do, will continue to do so long-term.
In general, AAirpass flights book into Y, J, or F with the exception of instant upgrades and plan ahead economy. On a standard 2 cabin domestic flight, IU books into A inventory. On 3 cabin transcon, IU books into D. Plan ahead Y books into H.
If an instant upgrade is offered on a given route, it is available to be booked anytime there is available inventory for sale, which can be monitored via Expert Flyer
Thanks for the info, this is helpful. Are you only able to see upgrades if you have an Airpass account or can they be found another way as well? Curious what determines availability on a route like CLT-FRA or any other one really...seems if that benefit is there it should be applied system-wide at least to some reasonable capacity. Obviously can't offer every seat in a cabin for upgrade, but to limit them to certain markets seems silly.