Melissa is out. She's pregnant. She can't do it in time.
They could fill in some obscure roles with those choices, but the iconic ones like C.J. are likely to be filled by people of a higher profile that fit the type. And frankly they should be because people are more likely to connect to that more. They'll feel like it's destiny to have those people in those iconic roles. More obscure actresses in the iconic roles wouldn't draw much interest.