If I understand you correctly, you want to have a subreport dynamically open with a specific report based on what the user selected. Is that correct?
If it is, it can't be done - there's no way to specify which report to open in a subreport from within Crystal. What you will have to do is add all of your possible subreports to the main report and use the suppression formula in the Section Expert and on each subreport itself to selectively suppress/show the subreports based on the data.
-Dell