Low end padding quickly crushes down to feel like nothing. The other issue is the carpet weight and density. Many times, both a 30oz/sqyd carpet is paired with a pad that is also low end in the upstairs areas by a builder. It looks adequate enough to make the sale. After the first year it becomes a joke. Replacement is the best long-term solution. Pad is not really important if you install a densely woven 55+ oz nylon carpet. Just don't expect as much from polyester or any even lower grade material.

If the carpet is in good shape, you might be able to simply upgrade the pad. I know someone who bought a house that had nice carpet - but the seller went cheap on the pad. They put a new pad under the carpet and it felt a lot better.