Hey @Ankur Bawiskar ,

Thank you for the response. I migrated my old dashboard using the Migration Center and it is now in Platform Analytics > Library > Dashboards. 
I selected this dashboard as the default dashboard in Dashboards template page in UI builder. However it still shows "Can't display this dashboard. The dashboard you were looking for was not found."

The post you sent looks pretty good! However, we are trying to migrate to the new experience as the old core UI functionalities might be limited in the future for new reports.