Cats are creatures of habit so it's highly unlikely you'll simply be able to swap their litter type. If they have been using the same type of litter for a long time, you'll have to help them get in the groove with HempyCat. They may may feel confused by the new litter and stop using it altogether, which isn't ideal.
Step 1 - Start with a fresh clean litter box
Mix in one-third HempyCat and two-third of your current litter
Step 2 - Once your cat takes to the mixed litter
Mix two-thirds HempCat to their current litter. Wait a few days!
Step 3 - Make the Switch
Once you notice your cat is has taken to step two, make the full switch. Use just HempyCat litter in their box.