That was weird, the KITTENS info popup had the conversion rate deleted - now it's back as before at 24 to 1. Maybe was an SE glitch. I assume the conversion stays fixed at this rate. The rate only changes when a new higher price for CATS is reached. I assume it stays fixed when the price falls, like now.
It means for those buying now, the daily interest remains the same 4%-ish, but for those who bought higher that has come down.