Rule out pain first, find where the threshold actually sits, then a gradual plan that keeps your animal below it — because above threshold, nothing is being learned. Plus the comfort advice most owners have been given backwards.
Pain, illness and sensory decline present as behaviour change — particularly in cats and older animals, and particularly when something appears suddenly. No amount of training fixes a sore joint, and starting with a behaviour plan can waste months.
That advice is widespread and it isn't supported. You cannot reinforce fear by reassuring a frightened animal — fear is an emotional state, not a behaviour being rewarded. The advice leads people to stand back from a panicking pet, which helps nobody.
Desensitisation only works below the point of panic. Above it, the animal isn't learning to cope — it's rehearsing the fear. Most plans fail from advancing too quickly, and it's read as the pet being untrainable.
It's often used alongside behaviour work from the start in moderate to severe cases, because an animal living above threshold can't learn. Owners frequently delay it out of guilt, and the animal spends longer distressed than it needed to.