Continuity mistake: Tony's hair length alternates between a short buzz cut and longer in different shots in the scene where Sil, Paulie and Chris visit him after his mother dies.
Continuity mistake: Livia's hair style keeps changing all the time while having her last talk with Tony. In this scene the Livia character has been superimposed from pre-existing footage as the actress Nancy Marchand had passed away.
Answer: That's not Jimmy, just a look-a-like. Jimmy's short and stout, the guy in "Christopher" episode is tall and medium built. The hair is also different.