The compiler does not guarantee strict backwards compatibility nor are there 9.0 options that provide this functionality (like a 8.0 compat switch).
Have you seen an issue where this is not the case?
I would expect 9.0 to be able to compile code that was compiled with 8.0 and if not, it may be worth an issue submission to premier support.
One thing to be aware of though, the default C++ libraries changed between 8.0 and 9.0 so you may see some differences there, but again it all depends.
Max
