does it require it? technically no. What the buyer won't see won't hurt them, right? 
Now me personally, I know what funkiness can enter into carpet and pads, so I'd want to change the pads. Think pet accidents, spilled allergens that soak into the pads, Potty training children etc etc.
It's a matter of what your conscience says. Me as a buyer, I'd want to know if the carpet AND padding were replaced as my family has a lot of allergies.
Now me personally, I know what funkiness can enter into carpet and pads, so I'd want to change the pads. Think pet accidents, spilled allergens that soak into the pads, Potty training children etc etc.
It's a matter of what your conscience says. Me as a buyer, I'd want to know if the carpet AND padding were replaced as my family has a lot of allergies.