Originally Posted by corporate-wage-slave

It is not impossible for that to happen, since the usual process is to put you in the lowest available fare bucket. However BA and AA take different approaches here. So if you were transferred at the codeshare level, you would still be in BA Basic since that isn't linked to a specific fare bucket - and you won't get free seat selection. If you were transferred at AA prime level then only fare bucket B is Basic Economy. As to what AA100 fare buckets are currently available, it depends on your Point of Sale, since they will vary between markets, but on a brief glance AA is still selling B on prime AA100 on 5 May.