Debbie, I believe that the terms and conditions in the booking allow Celebrity to make changes to the itinerary without requiring them to refund anything. However I would guess that if individuals asked for a refund because of a significant change in itinerary, Celebrity would grant it--they are just not going to advertise it. Also the further in the future the affected sailing is, the better as far as getting Celebrity to work with you.
As far as itinerary length changes go, I have had Celebrity cancel our 2021 Asia cruise (14 night Hong Kong to Singapore) and shift us to a shorter one (11 night, 2022)) and in that case they gave us a pro-rated refund for the 3 missing days. A year later, Celebrity cancelled the shorter one (11 night, 2022) and move us to a longer one (12 night, 2023) and they did not increase our fare.
David