Hi AC,
The script API method Ingo is referring to is assignDataSource. You can have one data source alias and then simply swap out the underlying actual data source with this based on your drop down selection. However, I'm not sure if the assignDataSource method is limited to only BW BEx Queries or if Universes are valid as well. The Design Studio Application Designer Guide is not clear on this. Perhaps you could try?
Regards,
Mustafa.