The current way of doing this is to make a copy of the dashboard and use ArcGIS Online Assistant to change the data source. See this discussion for more information: https://community.esri.com/thread/237825-how-to-duplicate-an-operations-dashboard
I followed your advice to edit the JSON, and change the web map that my Dashboard pointed to, that worked, but the Widget did not change, because they have different IDs, as I find these IDs in my new layer so that the Widget also changes, What do you advise me?
This stumped me as well. Seems like you should be able to copy/create a duplicate of an item from your Contents page.
Other's mentioned making a copy via the ArcGIS Assistant tool, however I found that while I could make a copy of almost every other Item, Dashboards were not selectable to copy.
The solution I found is in your dashboard select the drop down beside the "Save" button and select "Save As"