Change to a Live connection to the Power BI Service in existing Power BI Desktop file
It's great to have the live connection to Power BI Service but it only works on new blank file of Power BI Desktop :(
It would be VERY useful for existing Power BI Desktop files! Deleting all existing data sources and then connect to the online dataset (previously published, of course).
This way, we can conserve the reports and use the product more efficiently.
Josh Knight commented
This can be accomplished by deleting your original power query and connecting to a live connection data source via "get data"