It's not the studio, the performer makes that call (the studio obviously has input, since they like to coordinate campaigns). Christoph Waltz switched abruptly from campaigning in lead to supporting this year (which worked out nicely for him), dropping in on his costars who'd been there the whole time.
Sally Field is the most prominent actress in Lincoln
, but she is not an "actress in a leading role". The First Lady is clearly a supporting character. There have been any number of performances that blur the line between the two categories and where placement is arbitrary (Anthony Hopkins was thought to be a supporting actor in The Silence of the Lambs
, but he asserted that he wanted to be considered a lead). Helen Hunt would be a better instance of a lead actress positioning herself as supporting this year for strategic reasons.