I have never owned a dog, including cavaliers, that need consistency (IMHO read: boredom and poor nutritional variety :lol
in their food, nor slow changes in introducing foods. While domestication and pedigree breeding means all sorts of changes can take place in dogs internally as well as externally, dogs are not that far away from wild ancestors who would eat a huge variety of foods (or they wouldn't survive!) and I think most dogs not just tolerate but far prefer variety an d it is a healthier option as long as you read up a bit on canine nutrition and risky/lethal foods for dogs. There is info in the Cavalier Library section on feeding and the Rupert's Recipes booklet contains all sorts of things you can do too.
It is not even very healthy for a dog to get just a single source of good quality kibble all its life as nutritional needs vary thru life. Sure, we could all live just on fortified breakfast cereal (basically similar to a kibble in many ways!) but few would consider that a particularly healthy or interesting option! Kibble as is loses most of its true food nutrients in processing and they have to be added back in as supplements. So looking for good real food to add gives variety, and also, I would rotate kibble (eg when one bag is done, I'd opt for a different flavour or brand).
In 95% of cases, dogs become finicky and stuck on a tiny number of food sources because we make them that way by not feeding a variety of wholesome foods as a matter of course. There are a small number with digestive problems or which are problem eaters for various reasons but this breed in particular will eat practically anything, in unlimited quantities, given the choice, so introducing variety is pretty easy.
I DO however feel strongly that simply making up a diet based on whatever is read here or there on the net or what others say is NOT adequate and can even be a serious risk for some dogs and at least risks poor nutrition for many others. I think a good quality kibble therefore is an excellent basis for a good diet that covers the nutritional bases, or a good quality commercial food (eg commercial raw diet supplemented with other real foods. If going it alone, I recommend using information from someone like Monica Segal (she produces a booklet on cavalier homemade diets and she is a canine nutritionist).