Circle is 100% backward compatible with C++. That is a technical property of the language.
You are welcome to take your millions of lines of C++ code and it will compile without change using Circle as any valid C++ code is valid Circle code, which is the technical definition of being backward compatible.
You don't need to change existing code to use Circle or the new features Circle introduces, you can just write new classes and functions with those features and your existing code will continue to compile as-is.
You don't get the advantages of circle if you are constantly dealing with code that is returning raw pointers you have to deallocate. Or APIs where you need to pass in an index which the called function then uses vectors operator []. Safe C++ (from the same guy from what I can tell) only is safe if you used std2 containers, and otherwise rewrite your C++ entirely. Sure the world would be better if we did, but that would cost billions of dollars so it isn't happening. What we need is a way to introduce some safety into code that already exists without spending billions and a lot of time to rewrite it.
You are welcome to take your millions of lines of C++ code and it will compile without change using Circle as any valid C++ code is valid Circle code, which is the technical definition of being backward compatible.
You don't need to change existing code to use Circle or the new features Circle introduces, you can just write new classes and functions with those features and your existing code will continue to compile as-is.