Well I didn't do the "save as" trick but I did delete the old report and dataset from Power BI Service, then re-uploaded it again. New version works fine, however I had to recreate permissions (a pain) and I still have the old data sources listed. Even if "save as" works, it still seems like a bad solution because it is effectively a new report. All this to change a file pointer?
Please add 'delete' options for data sources to Power BI Service!