When using the assertion test step you can highlight multiple verifications and select "Change Source and Property". After you make your changes and select OK only the first verification is updated. ...
My use of the word bulk may have been a little misleading. My request is to be able to the source and property for multiple assertions at the same time. If you have an assertion test step with 20 assertions and you want to change 10 of them to use a different source; you will have to update each of them individually. Well it is possible to do this by updating the XML it would still require to update one property at a time.