Yes, you can change dates, ships and itinerary. The key word is "change" -- don't cancel the original placeholder reservation, you change it (to new dates, different ship/itinerary, etc.).
As a PP mentioned, a "placeholder" cannot actually be transferred. A placeholder is in itself a reservation, whether attached to a specific cruise or still open. That reservation number holds the placeholder benefits, which cannot be "transferred" to any other reservation number.