No characters are guaranteed at any meal, especially FC's. I'm sure it can be frustrating but Disney don't like to make promises they may not necessarily be able to keep.
I don't really think that is stated clearly on the website.
Cinderella makes your acquaintance in the majestic Grand Hall, before you ascend a spiral staircase to the banquet hall above. (A gilded elevator is also available to transport you to the festivities.)
Then, dine in a storybook setting surrounded by soaring stone archways, majestic medieval flags and spectacular stained-glass windows overlooking Fantasyland. During your time inside the castle,
visit with classic Disney Princesses.
Tiaras and glittering gowns are optional, although young Guests are encouraged to dress up for the occasion.
Character appearances are subject to change.
I would think they appearances subject to change would be the other princesses not the big one who is greeting you and you get your picture with included with cost:
As you enter Cinderella Castle, pose for a complimentary portrait with Cinderella herself. Your portrait will be delivered right to your table.
I am glad in my case it is a moot point b/c I've never had that experience, but I pay for CRT for Cinderella. I would be asking for a refund and not staying if Cinderella was not there.
Nowhere does it say in the event that Cinderella cannot appear she will be replaced with the Fairy Godmother or another friend. I think CRT is an exception where she should be there or people should be compensated in some way(allowed to cancel, discounted price). Again, you are paying and you are promised a photo with Cinderella...not Cinderella or one of her friends. I find it hard to believe with all the places Cinderella makes appearances in the MK they wouldn't be able to get a fill in(parade, FTH) I think Disney is in the wrong big time if they expect people to be satisfied with FG/friend when you are paying that kind of money. Another example where WDW is dropping the ball with CS.