afaik, there is no such function like copy/update whole list item.
neither sharepoint's APIs (web service, REST, CSOM) provide such an option...
Yes you can't update whole list item, instead you write a workflow in List A (to copy item to List B) like below.
1. Set workflow settings to allow workflow when items in 'List A' are modified/updated.
2. Find similar Item in 'List B' with some unique value and 'Delete' item from List B.
3. And 'Copy' current item from 'List A' to 'List B'.
So that you no need to configure all 40 items using 'Update/Create' item action.