Brian or someone will have to answer if it is actually possible.  I'm guessing that it is not possible with COBOL, as you would need some of the API's from .NET to accomplish it.

The much simpler solution, though it may go against a number of "best practice" habits, is to create a separate report for each data source.  It is a relatively simple matter to open a report and change its data source manually.  Then, save it as a separate instance of the report.  Have your COBOL simply call the appropriate report, rather than trying to modify the report's data source when it's called.

Now, obviously, this doesn't work if the name of your flat file changes between executions (e.g., includes the current date, or some version number).  In that case, you may need to get a little more creative.  I would recommend actually running a little operation to copy the data from the flat file into a common temporary file, generate the report based on the temporary file, then either clean or delete the temp file.  This will be the best way to keep Crystal happy.