As a holdover from the original pitch, Phineas is more flat and snarky toward Candace, compared to his overly chipper and excitable behavior in the rest of the show.
Doofenshmirtz's plan seems to be more worldwide, when he usually restricts it to just the Tri-State Area. His invention also does not end with "inator".
"Candace Loses Her Head" is the first episode to feature a musical number in production order.