Hill's suggestion is great if it's not for too terribly long.
But if it's like Face/Off where they switch for half the movie, I would just note they they have switched.
Ben will now be known as Marcy, because he is in Marcy's body and Marcy will now be known as Ben.
The reason I'd switch it this way (versus keep Ben as Ben even though he's in Marcy's body, because he's still Ben inside) is because other characters will be calling him Marcy, will be seeing him as Marcy and he will be played by the actress playing Marcy.
But again, if it's just a short period of time, then just note that Ben is in Marcy's body then that Ben looks in the mirror or whatever and he looks like himself again.
That all sounds really confusing, I hope it makes some sense.
I'd be interested to know though what you find out in those scripts. Please report back if you find any of them.