A cat with untreated diabetes, for example, drinks and urinates frequently, may overwhelm your efforts at keeping the box clean, and starts choosing other sites to relieve himself. A cat who suddenly starts biting may be in pain and is only lashing out to protect himself. A cat with a urinary- tract infection may find urinating painful and come to associate the pain with his litter box. All the behavior techniques in the world aren't going to cure a medical problem. You'll need your veterinarian's help for that.
Cats find change stressful and can react by altering their behavior to try to cope.
You need to calm your cat's stress in other ways, by limiting his territory for a while, for example, or putting him on medication. You need to look at your own role in any behavior problem. Your cat may not want to use the litter box you give him if it's rarely clean, for example, and asking him to leave the couch alone is really not fair if he has nothing else in the house to scratch. You need to provide him with some alternatives before you can hope for good behavior. Indoor cats need lots of things to keep themselves amused - lots of toys and lots of games, and lots of attention from you.