Yes, it is necessary because cats are quite attached to their old litter. If you suddenly change the litter they usually use, they may not be able to adapt, may not know how to use the toilet, or may urinate or defecate anywhere in the house.
The correct approach is to mix new and old litter. Start with 20% new litter, then gradually increase to 40%, 60%, and so on, until you're using only new litter. This will allow your cat to gradually accept the new litter.
