redhat Linux el3 to el5 C++ programs compatibility

[faridachandran]

Is there a white paper or any other documentation that would explain why we would show why the programs compiled on RedHat Linux el3 are not compatible with el5.

Are the C++ programs suppose to be compatible?

Is it correct to accept that the complied shared library .so on el3
would work instantly on el5.

thanks for reading my post,
Farida