Incremental Changes mean that you are making small changes to a listing, to improve it a little bit.

For major changes, you are expecting a bigger change of conversion, it can be either good or bad, so for poorly-converting listings it sometimes will be necessary to do major changes to it.

If a product is already the best seller, or generally selling very well, you will NOT WANT TO change the entire listing, instead a incremental change is recommended.

For improvement on ads, incremental or major changes to ADS you can try Samurai Seller here: https://www.samuraiseller.com