There are MAJOR differences in the underlying structure of the .rpt file starting with Crystal 9. Although it's possible to view older reports in the viewer that doesn't completely guarantee that the newer SDK will work with them.
Also, in Crystal 8 and earlier you could directly modify the SQL that Crystal generated. Reports where this was done will NOT work with the new SDK. They have to be completely rewritten using a Command (SQL Select Statement.) For example, in 2006 I spent a couple of weeks updating @150 reports to move from Seagate Info 7.5 to BusinessObjects XI r2 because we had modified the SQL to take the db owner name prefix off of the table names (Oracle database) to make it easier to move them between environments. Even something as simple a that cause the reports to lose the links between tables and do Cartesian joins instead of showing the correct data.
-Dell