SSRS stores the names of the columns as part of the report definition (rdl file), that's the only way it can know what to display where in the report itself. So if you save the report with the columns FirstName, LastName, and BirthDate, those are the only columns the report itself knows about. SO if you change the parameter and the stored procedure now returns CompanyName, ContactName, PhoneNumber the report can't know about those columns because they are not stored in the report definition (rdl file). If I were doing this I would probably write the stored procedure so that it always returns ALL the columns. In my example I would always return FirstName, LastName, BirthDate, CompanyName, ContactName, PhoneNumber, but the unused columns would be NULL.