Apart from Heredia, the Power also added RHP Clario Perez and IF Chris Diaz from extended spring training. They replaced Stetson Allie, Orlando Castro and Joely Rodriguez, who were all moved up.
Perez is fairly similar to Isaac Sanchez; he's a smallish righty who throws pretty hard, generally 92-94 (practically all their young pitchers throw at least in the low-90s these days), and has a potentially good curve. He's also had command problems. He probably profiles as a reliever, but made a few starts at State College last year.
Diaz played at State College after being drafted last year. He didn't hit much and probably profiles as a utility infielder.
The Pirates promoted Jordan Steranka a couple days ago. That's more promotions to the Power than from the Power, so I'm not sure exactly how it works out. Anyway, Steranka would have signed quickly after the draft last year, but couldn't pass a physical because he had mono. He ended up signing in September, so he'll make his pro debut with the Power. He'll probably play first, third and left, and could get a lot of Allie's time at first.