Have you tried opening the .pbix file from PowerBi.com/GetData? I've found changes to connections when published via PBI Desktop don't always get updated in PBI Service. Opening the file via PowerBI.com seems to force the service's copy of the connection to be refreshed with any changes.
Yes, that's what I meant... Go to PowerBI.com, click on "Get Data", go to "File" and open your .pbix. Opening the file in this manner seems to fix similar issues for me. I was connecting to a Web API rather than Postgres. PBI wouldn't update the credentials via online "Data source credentials". Opening the .pbix file from within the online service seemed to do the trick.