yes, if it makes them nervous. the same thing happens when you bag fish for shipping, which is why most keepers advocate catching fish into a bucket first, then transferring them into bags from there. i have no clue, why my cat feels the need to do the same thing, after replacing the sand in her litter box. :roll:
Same thing happens when we clean the cat boxes and the horse stalls. As soon as they are clean, they feel the need to mess in it again. lol
Not sure why they poo right after a water change, could be nervousness, could be something else. I wouldn't worry about it unless the were acting funny - swimming erratically, not eating, breathing hard, etc.
My cat will stand by me every morning and evening watching me clean his litter box so he can jump right in there and pee in the clean stuff. LOL. When I clean the horse stalls, I usually go back through with an empty wheel barrel to pick up the new piles. lol never fails. At least half of them can't resist going in their clean stall.