you will not able to change it back to database mode if you dont have this Data source in your data source settings ( for exmaple data source setting from function)
and always only get error
also if you change mode from database to integrated only for one database on same ip, pbi sets integrated mode to whole same ip databases, but when you restoring from integrated (as only one available method, below)to database pbi setting only this mode for only this database!!!
also you cant create new data source on same database, you need create connector with different working database first and when change source in query editor, so if you have 100 databases, you need manually create each database and edit sources to change mode