If you've only had them neutered, then I wouldn't worry about changing litters. Kitten neuters almost never get infected. I'm not sure why your vet would recommend changing litter, since whatever litter you had been using isn't going to cause a problem and changing litters could actually cause them to stop using the litter box and develop behavioral issues with the box. In fact, changing litters is likely enough to cause litter box problems that there are many vets out there that don't even recommend changing litters if the cat is declawed (although probably many more recommend using paper litter for about a week after the declaw). I've assisted in hundreds of kitten neuters, and none of the vets I've talked to have ever recommended using paper litter after a neuter or spay. And honestly, I have never seen a single neuter incision get infected. And the few times I have seen a spay incision get infected it was because the cat messed with it, not because of litter sticking to it.