There are notes about this elsewhere on the site as it has come up a few times, but in general many dogs (not only cavaliers) have a slightly lower metabolism once neutered (this is generally seen as a favorable result because you end up with a less hyper and more attentive dog). It does mean that you need to decrease calories–whether you are feeding a mix of wet and dry food (which, personally, I feel is nicer for the dog then to eat dry food day in and day out) doesn't matter, it's the level of caloric intake. Given that many people neuter their dogs around the time that they are becoming adults also means many people are overfeeding their dogs anyway, as puppies are growing and need the extra calories. Many adults switch to eating about a third less than they did as puppies but many owners keep feeding the same amount and their dog begins to gain weight regardless of whether it is neutered. Cavaliers generally are also prone to overeat and gain weight and often start to do this after puppyhood if food intake isn't carefully measured & monitored (don't rely on eyeballing an amount of food -- many people are surprised to see how much less their dog is supposed to be eating when the amount is measured. Most use dog bowls that are huge for a cavalier, too. I use small steel cat bowls and they work well!).
So I would reduce the amount of food you are giving overall by a third and keep a very close eye on treats which can be very high in calories. Things like dental sticks or pigs ears can have the caloric equivalent of entire day's meal. In family situations, it's often the case that other family members continue to give treats even though you think the dog isn't getting them or is only getting a few. It is much better to train your dog to enjoy low calorie treats like pieces of fruit or carrots (but be very careful about potentially fatal items like grapes or raisins. There are lists of poisonous foods in the library section). or, just get used to giving a single treat before bedtime, and always remember to reduce an entire day's food intake by the same amount that has been given as training treats when you are training your dog. In general, a dog doing an obedience class probably does not need to be fed before going and will not need to be fed that day as they will get pretty much the equivalent of their entire day's meal from rewards use during the training class.
It is very hard to tell you what has caused the change in her coat. A thin flat coat is more typical of puppies while a fuller coat would begin to come in at around the age she is, so there would already be an expected change. Also, obesity tends to cause coat changes too, often they become drier and curlier. Neutering can sometimes cause mild coat changes as well. So can excessive washing of the coat, or having groomers shave the dog (breeders consistently say this causes the coat to come back in curlier and often in a more cottony state). And of course, genetics plays a role. Some dogs develop curlier coats than others–curly coats are actually quite common.
The best thing to do is to reduce her food intake and to maintain or increase her exercise level. Most dogs actually need far more exercise than we give them–a walk once or twice a day will hardly dent their energy level and most walks are not very active from the dog's point of view–a dog can walk about three times faster then we would so they are almost always having to pace themselves far more slowly than would be their ideal (speeding up closer to dog speed is actually a good way for the humans to stay in shape!). Real activities, like a good half an hour of playing fetch, doing an agility class once a week, a long active hike up in the mountains or along the beach where they can run and explore–and of course brain challenging activities also exhaust them–these are all good ways of giving the dog mental and physical exercise that will help keep them in shape.
In general, you should be able to see a distinct waist on your dog when viewed from above and should be able to easily feel their ribs, although they should not feel bony. My own vets note that 90% of the Cavaliers that they see are overweight and that most owners think their dog is in shape when actually it is fat. So it is worth having a conversation with a vet about exactly how much weight a dog needs to lose. That said, they tend to then recommend diet dog foods, which I think are a waste of money–it is only just filler and simply cutting back on a heavy hand when feeding and giving treats will bring the same result, generally at less personal expense.
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