S4: Twilicorn replaces Celestria, not in position but in screen time on the show. Meaning Twilicorn gets phased out of the mane 6. Gets replaced by .....the great....and powerful...TRIXIE.
Rainbow Dash get a full time gig with the Wonderbolts and gets phased out. Gets replaced by Lightning Dust.
Rumble joins the CMC...enter sea ponies: "shoobie doo doo doobie doo"