I've "ported" ~200k lines of C++03 to C++11 and so far I've had to make exactly one change due to language changes (C++11 forbids narrowing conversions in braced inits, while C++03 just did nonsensical things). That's not to say that no other changes were required, but the rest were things like fixing code that had both C++03 and C++11 versions and the C++11 version was broken, and fixing compatibility issues with libc++.
