Bulk update datasource type

Few questions:

Does each report have it's own dataset?

Do you premium capacity or ppu?

Not sure how easy it is (never tried) but if you have premium you can connect to a workspace via an xlma end point and interact the tabular object model (TOM) with a c# script. You'd have to loop through each workspace/dataset in turn though. 

Have a read of

https://docs.microsoft.com/en-us/analysis-services/tom/tom-pbi-datasets?view=asallproducts-allversions