There is no difference to the project file format in SuperCard 4.8. Your current projects will run in 4.5, 4.6, 4.7, and 4.8. Of course if you use features of newer versions, opening those updated projects in older versions will result in errors (newly added controls will not draw with the new style, and new scripting additions will result in script errors). There are also a number of existing externals that have been updated for 4.8 which should be replaced. These are defined in the "SuperCard 4.8 New Features and Additions" document, as well as in the XTend tome of the SuperCard Help System.
As far as standalones go, you can rebuild those using your existing "Make files" with the new Standalone Maker. As is the SuperCard tradition, we have gone out of our way to maintain as much backward compatibility as possible, while keeping the upgrade process to newer versions painless.