I went the simular process and problems a few weeks back with tying to do a class of service upgrade.
What i have found out is that each agent will tell you differently. Why? Well it is because they do not want to help with the leg work yo do so.
Finally an agent told me. You need to book an upgradable fair with aa ba or ib. Meaning full fair. Y W etc. But when booking that more expencive fair be careful. Upgrades are subject to availability. But fyi AA has a 24 hr cancelation window for metal holders.
My bookings AA bos-dub swu. Return dub-bos on BA 1st with miles.. fair was $480 agravation wasnt worth it.
Hope this helps