There are many past threads on this topic which might be good to search. First --
all puppies lose their soft puppy coats (often they come out all at once -- 'blowing' their coat is the phrase -- and can look pretty ratty til the adult coat comes in!) so the issue of neutering is separate to that.
In general -- some dogs are affected, some are not. The change at any rate, even when it does happen in some dogs, is a very minor cosmetic element and most people would not even know the difference. I think most people mistake normal changes from puppy coat to adult coat for a neuter coat, myself, when it is simply due to the age of the dog and the fact it coincides with when people often neuter -- most puppies lose their really soft fine coat for a coarser adult coat around 9-18 months. Some dogs genetically have a coarse, drier and/or curlier coat as well. Some colours -- tris and B&Ts with black -- often have thicker, coarser, heavier coats. Extra weight or trimming the dog often produces these kinds of coat changes. MOST cavalier I see are overweight and I have seen lots of unspayed females in particular that have dry curly coats likely due to being overweight. Diet can affect coat. Genes play the dominant role.
Many -- me included -- would argue the health, behaviour and related benefits of neutering outweigh leaving them intact, for reasons noted under a thread in the Library section. This is especially true for females, which run a very significant risk of mammary cancer over their lifetime if left unspayed (1 in 4 chance) but even more of a concern perhaps in this breed -- statistically around a 40% risk of the often fatal, expensive to treat uterine infection pyometra, which is often not spotted until it is too late for the dog to be saved and is extremely distressing for the dog (and owner). Cavaliers have amongst the highest rate of any breed for this infection and its risk is one of the key reasons vets argue for spaying.
Of the five cavaliers I have owned, all neutered, only Lily has a poor cottony coat and she already had that when I got her from the pound; Kept neatly trimmed, she hardly looks any different from the others or any of the intact females around my area (I bought a handy thinning shears for her coat and funny thing is, many people think she is the cutest in part BECAUSE of her fluffy coat! :lol
. The intact tri female up the street had a far curlier and coarser coat than any of the other four have ever had as did two rubies owned by someone who kept both (M and F) intact to show. My males have very straight, silky coats and both were neutered before 12 months. Genetically their lines are known for straight, not very heavy coats. Lucy (blenheim) had a very thick, wavy coat and she had that before she was spayed (as a mature adult). Tansy (B&T) has a long and heavy coat, that is no different really from before she was spayed (again as an adult). Lots of cavaliers naturally have wavy or curly coats -- it makes not one jot of difference unless you are showing the dog in which case you can dry them out to be straighter but IMHO it is a bit of a stupid element of the breed standard as probably most cavaliers are at least wavy coated to some degree. I think wavy coats look lovely and full.
In general simply using a conditioner that has a form of silicone (any ingredient ending in '-one') in it will give a nice shine and finish to a dog's coat. Finishing conditioners that you spray on and do not wash out and also help the coat repel dirt.