Answering my own question, for anyone else who needs to know. The answer is yes, you have to go through the whole process again, including criminal background checks from every state you've lived in, worked in, or went to school in. (Alternatively, just the state you live in and then an FBI criminal history check. The BON automatically submits your info to the Pennsylvania State Police system for a background check when you submit your application.)
Pennsylvania treats a second specialty as a new, entirely separate license. There is no process for speeding things up because you are already licensed with your first specialty. Also, your prescriptive authority is tied to a specific specialty. So if you are going to prescribe medications with your new specialty, you will need to do an additional prescriptive authority application.