I'm a psychologist trained in behavior modification and I assure you that you can use behavior modification techniques with pets, much like humans. My psychologist friend (we are an odd bunch) trained her cat to use a toilet by gradually fading away the litter box that she placed on top of the toilet until the cat used the toilet independently, no lie. So here is what I would recommend trying:
1. Rule out medical issues or new stressors in the cat's life. It's always important to find out what has changed in the cat's environment that could be contributing to this. It could very well be the change in the litter.
2. Reinforce the cat when they use the litter box to go number 1. You can do this by immediately giving the cat a treat. I would use a treat that the cat does not have access to at any other time during the day.
3. Restrict access to areas of the house where your cat has gone number 2. You may have to close your cat in the bathroom or another small room with the litter box, so that they are not tempted to go in other locations and so that they get used to going in the litter box again.
Sorry for the lengthy reply, hope it works out.