We booked our up coming cruise online a year ago.
Upgrading from classic to premium beverage package. We paid for an upgrade on the website. Months later there was a 50% off sale. We cancelled our first upgrade then purchased again at 50% off, again on the website. Same as shore excursions, we rebooked when there was a 40% off sale.
Cruise fare - there were big price drops months after we booked. Phoned Celebrity and they repriced our cruise for us twice - saved us thousands. You can do this before final payment is made.
Specialty dining - the website says it is cheaper to buy before you get on board so who knows. On full sailings specialty dining can get booked out.
Where are you from to? π