I don't think there's any trick, Doctor Who is usually quite up front about revealing these things themselves, unless there's a specific reason to keep the surprise. Apparently he's appearing as The Master in the same episode as Missy, so I think we're just going to get a cool multi Master story, that may act as closure for Simm to regenerate into Missy, since that was never explained.
Whoa, now that I think about it, Simm's Master was stuck at the end of the time war. Capaldi's first on screen appearance was technically in Day of the Doctor helping the other Doctors end the time war. Maybe we're finally going to see how Capaldi got involved in Day of the Doctor (which I've been waiting for since day 1), and see how Simm escaped the time war and became Missy. Kill two birds with one stone in a big send off for Capaldi. The brief shot of Simm in the trailer definitely appears to be the time war, so it makes sense. The last we saw of Missy, she was aligned with the Daleks. She could also be involved in the time war.
Also, I don't know if a new Doctor has even been cast yet, but I get the feeling we won't see Capaldi's full regeneration until the Christmas special. I think the 12th episode will end with the regeneration process just starting.