Compatibility of classes in DLL's

I have a problem regarding classes in DLL's. If i change something in one of my basic classes, like adding a virtual function, it is necessary to compile all programs with the new header file of the class definition. So it is a big problem to handle this. I had to recompile each old program.

Is there a solution for such a problem ? How does MS handle this in MFC ? As i have seen, MFC42 has changed from VC5.0 to VC6.0 and the basic classes like CWnd get new members and virtual functions, but old programs compiled with VC6.0 are working. How this ?