If initially both parties are self-represented litigants, and both obtain counsel, will counsel be able to file a substitution in a p2e case?

Yes. Cases in which both the appellant and respondent are self-represented litigants will be converted into P2E cases. If one of the parties retains counsel after a P2E case has been created, the attorney would file a substitution of attorney within the case in eCourts Appellate. See FAQ How do I file a substitution of attorney for instructions.